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Formal proof · Proof and reasoning

Coordinate Proofs

A landscaper stakes out a patio on a grid where 1 unit is 1 foot. The four stakes are at $(0, 0)$, $(12, 5)$, $(7, 17)$, and $(-5, 12)$. She has no protractor. Is the patio a rectangle? Could it even be a square? By the end of this page you will prove it using only numbers.

A picture can fool you. Two sides that look equal might differ by a hair, and an angle that looks square might not be. A coordinate proof puts the figure on a coordinate plane and uses algebra to settle the question exactly.

Distance and midpoint

Draw a right triangle under any segment. The horizontal leg is $x_2 - x_1$ and the vertical leg is $y_2 - y_1$. The segment is the hypotenuse, so the Pythagorean Theorem gives its length.

Segment from A(-2, 1) to B(4, 5) with a right triangle showing run 6 and rise 4, and its midpoint M(1, 3) A(−2, 1) B(4, 5) run = 6 rise = 4 M(1, 3) d = √52 = 2√13
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A circular irrigation system is mapped on a grid where $1 \text{ unit} = 10 \text{ feet}$. The endpoints of a diameter are located at $P(-4, 3)$ and $Q(2, -5)$. Use the…

A circular irrigation system is mapped on a grid where $1 \text{ unit} = 10 \text{ feet}$. The endpoints of a diameter are located at $P(-4, 3)$ and $Q(2, -5)$. Use the midpoint and distance formulas to find the coordinates of the center of the circle and the exact length of its radius in grid units.

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A community garden is shaped like a kite with vertices at $A(0, 3)$, $B(4, 6)$, $C(8, 3)$, and $D(4, -2)$. Calculate the exact total perimeter of the garden.

A community garden is shaped like a kite with vertices at $A(0, 3)$, $B(4, 6)$, $C(8, 3)$, and $D(4, -2)$. Calculate the exact total perimeter of the garden.

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In triangle $ABC$ with vertices $A(-4, 1)$, $B(2, 5)$, and $C(4, -3)$, a median is drawn from vertex $B$ to side $AC$. Find the exact length of this median.

In triangle $ABC$ with vertices $A(-4, 1)$, $B(2, 5)$, and $C(4, -3)$, a median is drawn from vertex $B$ to side $AC$. Find the exact length of this median.

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Slope: parallel and perpendicular lines

Slope measures steepness: $m = \dfrac{y_2 - y_1}{x_2 - x_1}$, which is rise over run. Slope tells you how two lines relate.

Why opposite reciprocals? Rotating a line $90^\circ$ swaps its rise and run and flips one sign. A "rise 1, run 2" line ($m = \tfrac12$) turns into a "rise 2, run $-1$" line ($m = -2$).

Segments EF with slope 1/2 and GH with slope -2 meeting at H with a right angle E(0, 0) F(4, 2) G(1, 3) H(2, 1) m = 1/2 m = −2
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Two straight hiking trails in a national park are plotted on a map. Trail 1 passes through the points $A(-2, 1)$ and $B(2, 9)$. Trail 2 passes through the points…

Two straight hiking trails in a national park are plotted on a map. Trail 1 passes through the points $A(-2, 1)$ and $B(2, 9)$. Trail 2 passes through the points $C(1, -3)$ and $D(4, 3)$. Prove algebraically that the two trails are parallel.

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On a city planning grid, Main Street connects the points $M(0, 5)$ and $N(4, 2)$. Oak Street connects the points $O(-3, -2)$ and $P(0, 2)$. Use the slope criteria to…

On a city planning grid, Main Street connects the points $M(0, 5)$ and $N(4, 2)$. Oak Street connects the points $O(-3, -2)$ and $P(0, 2)$. Use the slope criteria to determine if these two streets intersect at a right angle.

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Proving facts about triangles

Each triangle type has a coordinate test:

To prove Show
Isosceles two sides have equal length
Equilateral all three sides have equal length
Right two sides have slopes with product $-1$, or $a^2 + b^2 = c^2$ for the side lengths (converse of the Pythagorean Theorem)
Triangle PQR with vertices P(1, 1), Q(5, 3), R(3, 7) and squared side lengths 20, 20, 40 P(1, 1) Q(5, 3) R(3, 7) PQ² = 20 QR² = 20 PR² = 40
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Prove algebraically that the triangle with vertices $A(1, 2)$, $B(4, 6)$, and $C(7, 2)$ is an isosceles triangle by calculating the lengths of its sides.

Prove algebraically that the triangle with vertices $A(1, 2)$, $B(4, 6)$, and $C(7, 2)$ is an isosceles triangle by calculating the lengths of its sides.

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A triangular plot of land has corners at $D(0, 0)$, $E(3, 4)$, and $F(7, 1)$. Prove that the plot is a right triangle by showing that two of its sides are perpendicular.

A triangular plot of land has corners at $D(0, 0)$, $E(3, 4)$, and $F(7, 1)$. Prove that the plot is a right triangle by showing that two of its sides are perpendicular.

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A surveyor maps three historical landmarks at $P(-5, 2)$, $Q(1, 4)$, and $R(3, -2)$. Prove that the triangle formed by these landmarks is an isosceles right triangle.

A surveyor maps three historical landmarks at $P(-5, 2)$, $Q(1, 4)$, and $R(3, -2)$. Prove that the triangle formed by these landmarks is an isosceles right triangle.

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Proving a quadrilateral is a parallelogram

You can pick any one of four tests. Choose the one that needs the least work.

The diagonal test is often the shortest, since it takes just two midpoints. A rectangle is a parallelogram with one right angle, so first prove parallelogram, then check one pair of adjacent sides for slope product $-1$.

Quadrilateral EFGH with diagonals crossing at their common midpoint M(3.5, 0.5) E(0, 2) F(5, 3) G(7, −1) H(2, −2) M

Missing vertex. In a parallelogram $ABCD$ the diagonals $\overline{AC}$ and $\overline{BD}$ share a midpoint. So find the midpoint of the diagonal you know, then reflect the known endpoint of the other diagonal through it.

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Quadrilateral $PQRS$ has vertices $P(-1, 1)$, $Q(3, 2)$, $R(2, -2)$, and $S(-2, -3)$. Prove that $PQRS$ is a parallelogram by demonstrating algebraically that its…
X Y P Q R S

Quadrilateral $PQRS$ has vertices $P(-1, 1)$, $Q(3, 2)$, $R(2, -2)$, and $S(-2, -3)$. Prove that $PQRS$ is a parallelogram by demonstrating algebraically that its diagonals bisect each other.

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A rectangular foundation for a new building is staked out at points $A(-3, 1)$, $B(1, 4)$, $C(7, -4)$, and $D(3, -7)$ on a site map. Prove that quadrilateral $ABCD$ is a…
X Y A B C D

A rectangular foundation for a new building is staked out at points $A(-3, 1)$, $B(1, 4)$, $C(7, -4)$, and $D(3, -7)$ on a site map. Prove that quadrilateral $ABCD$ is a rectangle by showing it is a parallelogram and has at least one right angle.

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Three vertices of a proposed parallelogram-shaped plaza are $A(-2, -1)$, $B(1, 3)$, and $C(6, 2)$. Find the coordinates of the fourth vertex $D$ (assuming the vertices…
X Y A B C D(?, ?)

Three vertices of a proposed parallelogram-shaped plaza are $A(-2, -1)$, $B(1, 3)$, and $C(6, 2)$. Find the coordinates of the fourth vertex $D$ (assuming the vertices are in the order $ABCD$). Then, prove algebraically whether or not the plaza is a rhombus.

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Rhombuses, squares, and trapezoids

Each special quadrilateral has its own checklist.

Shape What to show
Rhombus all four sides congruent (or a parallelogram with perpendicular diagonals)
Rectangle a parallelogram with a right angle (or a parallelogram with congruent diagonals)
Square four congruent sides and a right angle (rhombus + rectangle)
Trapezoid exactly one pair of parallel sides
Isosceles trapezoid a trapezoid whose legs (the non-parallel sides) are congruent
Kite two pairs of adjacent congruent sides
Rhombus ABCD with all sides 5 and perpendicular diagonals A(0, 0) B(3, 4) C(8, 4) D(5, 0) 5 5 5 5
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A state park boundary forms a quadrilateral with vertices $W(-2, -1)$, $X(1, 3)$, $Y(6, 3)$, and $Z(3, -1)$. Prove algebraically that the park is a rhombus, and show…

A state park boundary forms a quadrilateral with vertices $W(-2, -1)$, $X(1, 3)$, $Y(6, 3)$, and $Z(3, -1)$. Prove algebraically that the park is a rhombus, and show that it is not a square.

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Prove that the diagonals of the quadrilateral with vertices $K(-3, 2)$, $L(2, 4)$, $M(4, -1)$, and $N(-1, -3)$ are both perpendicular and congruent. Based on these…

Prove that the diagonals of the quadrilateral with vertices $K(-3, 2)$, $L(2, 4)$, $M(4, -1)$, and $N(-1, -3)$ are both perpendicular and congruent. Based on these properties, what specific type of quadrilateral is $KLMN$?

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Given quadrilateral $QRST$ with vertices $Q(-5, 1)$, $R(-1, 4)$, $S(5, 4)$, and $T(9, 1)$. Prove that $QRST$ is an isosceles trapezoid by demonstrating that it has…

Given quadrilateral $QRST$ with vertices $Q(-5, 1)$, $R(-1, 4)$, $S(5, 4)$, and $T(9, 1)$. Prove that $QRST$ is an isosceles trapezoid by demonstrating that it has exactly one pair of parallel sides and that its non-parallel sides are congruent.

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The midpoint quadrilateral

Take any quadrilateral and connect the midpoints of its sides in order. The new quadrilateral is always a parallelogram.

Quadrilateral ABCD with the parallelogram formed by joining the midpoints of its sides A(0, 0) B(4, 2) C(6, 8) D(−2, 6)

Why it works. $\overline{PQ}$ joins the midpoints of two sides of $\triangle ABC$, so it is parallel to $\overline{AC}$ and half as long. Check: $AC = \sqrt{36 + 64} = 10$ and $PQ = \sqrt{9 + 16} = 5$ ✓. In the same way, $\overline{QR}$ is half of diagonal $\overline{BD}$ and parallel to it. So the sides of the midpoint quadrilateral are parallel to the diagonals and half as long. This tells you what shape to expect:

  • If the diagonals of the big quadrilateral are congruent (a rectangle, for example), all four sides of $PQRS$ are congruent, so $PQRS$ is a rhombus.

  • If the diagonals are perpendicular (a rhombus, for example), adjacent sides of $PQRS$ are perpendicular, so $PQRS$ is a rectangle.

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Points $A(-4, -2)$, $B(-2, 4)$, $C(4, 2)$, and $D(2, -4)$ form a quadrilateral. Prove that the midpoints of its four sides, when connected in order, form a parallelogram.

Points $A(-4, -2)$, $B(-2, 4)$, $C(4, 2)$, and $D(2, -4)$ form a quadrilateral. Prove that the midpoints of its four sides, when connected in order, form a parallelogram.

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Prove that the points $A(1, 4)$, $B(4, 5)$, $C(5, 2)$, and $D(2, 1)$ form a rectangle. Next, find the midpoints of its four sides and prove that the quadrilateral formed…

Prove that the points $A(1, 4)$, $B(4, 5)$, $C(5, 2)$, and $D(2, 1)$ form a rectangle. Next, find the midpoints of its four sides and prove that the quadrilateral formed by connecting these midpoints in order is a rhombus.

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Perpendicular bisectors and intersections

The perpendicular bisector of a segment passes through its midpoint at a right angle. Every point on it is the same distance from both endpoints, and every point equidistant from both endpoints lies on it.

To write its equation:

  1. Find the midpoint of the segment.

  2. Find the slope of the segment and take its opposite reciprocal.

  3. Write the line in point-slope form, $y - y_1 = m(x - x_1)$, using the midpoint.

Segment AB from A(-1, 2) to B(5, 6) and its perpendicular bisector through midpoint M(2, 4) A(−1, 2) B(5, 6) M(2, 4) y = −1.5x + 7

Intersection of two lines. Write both lines as $y = mx + b$. (For a line through two points, find $m$ first, then plug in one point to get $b$.) Set the right-hand sides equal, solve for $x$, then find $y$. Different slopes mean exactly one intersection point.

To show two lines are neither parallel nor perpendicular, compute both slopes and note that they are not equal and their product is not $-1$.

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A new cell tower is built at point $T(3, 4)$ to serve a rural area. Two houses are located at $H_1(-1, 1)$ and $H_2(7, 1)$. Prove algebraically that the cell tower lies…

A new cell tower is built at point $T(3, 4)$ to serve a rural area. Two houses are located at $H_1(-1, 1)$ and $H_2(7, 1)$. Prove algebraically that the cell tower lies on the perpendicular bisector of the segment connecting the two houses by showing it is equidistant from both.

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Find the equation of the perpendicular bisector of the segment joining $M(-2, 5)$ and $N(4, 1)$. Then, verify algebraically whether the point $P(3, 6)$ lies on this…

Find the equation of the perpendicular bisector of the segment joining $M(-2, 5)$ and $N(4, 1)$. Then, verify algebraically whether the point $P(3, 6)$ lies on this perpendicular bisector.

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Two straight natural gas pipelines are laid out on a coordinate grid. Pipeline 1 connects $A(1, 8)$ and $B(7, 4)$. Pipeline 2 connects $C(1, 3)$ and $D(7, 9)$. Prove…

Two straight natural gas pipelines are laid out on a coordinate grid. Pipeline 1 connects $A(1, 8)$ and $B(7, 4)$. Pipeline 2 connects $C(1, 3)$ and $D(7, 9)$. Prove that the pipelines are neither parallel nor perpendicular, and find the exact coordinates of their intersection point.

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Triangle centers

Three lines in a triangle each meet at one point. You find that point by intersecting two of the lines, then you verify with the third.

Center Lines Meaning
Centroid medians (vertex to midpoint of opposite side) average of the three vertices
Circumcenter perpendicular bisectors of the sides equidistant from the three vertices
Orthocenter altitudes (vertex perpendicular to the opposite side) where the altitude lines meet

The centroid has a shortcut: $\left(\dfrac{x_1 + x_2 + x_3}{3}, \dfrac{y_1 + y_2 + y_3}{3}\right)$.

Triangle with vertices A(0, 0), B(6, 0), C(2, 6) and its circumscribed circle centered at the circumcenter (3, 7/3) A(0, 0) B(6, 0) C(2, 6) O(3, 7/3)
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For a triangular nature reserve with vertices $U(-2, 4)$, $V(5, 5)$, and $W(6, -2)$, find the coordinates of the circumcenter $C$ by determining the intersection of the…
X Y U V W C

For a triangular nature reserve with vertices $U(-2, 4)$, $V(5, 5)$, and $W(6, -2)$, find the coordinates of the circumcenter $C$ by determining the intersection of the perpendicular bisectors of two sides. Then, prove algebraically that this point is equidistant from all three vertices.

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Given triangle $ABC$ with vertices $A(-20, 15)$, $B(0, -25)$, and $C(24, 7)$. Find the exact coordinates of the orthocenter by determining the equations of two altitudes…

Given triangle $ABC$ with vertices $A(-20, 15)$, $B(0, -25)$, and $C(24, 7)$. Find the exact coordinates of the orthocenter by determining the equations of two altitudes and finding their intersection point. Verify algebraically that the third altitude also passes through this exact point.

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A new high-speed rail station is being planned. The planners want it to be equidistant from three major tech hubs located at $A(-2, -2)$, $B(5, -1)$, and $C(-3, 5)$ on a…

A new high-speed rail station is being planned. The planners want it to be equidistant from three major tech hubs located at $A(-2, -2)$, $B(5, -1)$, and $C(-3, 5)$ on a regional map (where $1 \text{ unit} = 10 \text{ miles}$). Find the exact coordinates of the station. Then, prove algebraically that the line connecting the station to the midpoint of side $AB$ is perpendicular to $AB$.

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Proofs with variables

Checking one triangle with numbers does not prove a theorem for all triangles. For a general proof, use letters for the coordinates.

Place the figure smartly. Put one vertex at the origin and one side on an axis. When midpoints are coming, use even coordinates such as $2a$ and $2b$, so halves come out clean.

Rectangle with vertices (0, 0), (w, 0), (w, h), (0, h) and its two diagonals (0, 0) (w, 0) (w, h) (0, h)

For the same strategy on a midsegment, compute the two midpoints, then use slope for "parallel" and distance for "half as long." When a problem has a free point $P(x, y)$, keep $x$ and $y$ as letters and expand each squared difference carefully.

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Use a coordinate proof to show that the segment joining the midpoints of two sides of a triangle is parallel to the third side and is half its length. Place the…

Use a coordinate proof to show that the segment joining the midpoints of two sides of a triangle is parallel to the third side and is half its length. Place the triangle's vertices at $P(0, 2a)$, $Q(2b, 0)$, and $R(2c, 0)$ to facilitate your algebraic proof.

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A drone flies over a rectangular field. Prove algebraically that for any rectangle centered at the origin with vertices $(a, b)$, $(-a, b)$, $(-a, -b)$, and $(a, -b)$,…

A drone flies over a rectangular field. Prove algebraically that for any rectangle centered at the origin with vertices $(a, b)$, $(-a, b)$, $(-a, -b)$, and $(a, -b)$, the sum of the squares of the distances from any arbitrary point $P(x, y)$ to the four vertices is equal to $4(x^2 + y^2 + a^2 + b^2)$.

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Back to the patio

Now answer the landscaper's question. Stakes: $A(0, 0)$, $B(12, 5)$, $C(7, 17)$, $D(-5, 12)$.

Summary

  • A coordinate proof uses exact algebra on a plane. Plot, pick the right tool, compute exactly, then conclude with a definition or theorem.

  • Distance $\sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$ shows congruent sides. Midpoint (average the coordinates) shows bisecting. Slope shows parallel ($m_1 = m_2$) and perpendicular ($m_1 m_2 = -1$).

  • Triangles: isosceles by two equal sides, right by perpendicular slopes or $a^2 + b^2 = c^2$.

  • Quadrilaterals: parallelogram via slopes, side lengths, or diagonals with the same midpoint. Add a right angle for a rectangle, four equal sides for a rhombus, and both for a square. A trapezoid has exactly one parallel pair, and it is isosceles if its legs are congruent.

  • Perpendicular bisector: midpoint plus opposite-reciprocal slope. Intersections come from solving $y = mx + b$ equations. Circumcenter, orthocenter, and centroid come from two lines, and you verify with the third or with distances.

  • For general proofs, use letters (such as $2a$, $2b$) for coordinates, and never rely on one numerical example.

See all the exercises in this section