"Solving Non-Right Triangles: Law of Sines and Law of Cosines"
Emma and Marcus leave the same campsite in Yellowstone on two straight trails that form a $70^\circ$ angle. Emma hikes 3 miles and Marcus hikes 4 miles. How far apart are they? There is no right angle anywhere, so $\sin$, $\cos$ and $\tan$ on their own won't do it. You need a tool for any triangle.
Which Law Do I Use?
Label every triangle the same way: vertices $A$, $B$, $C$, and each side named with the lowercase letter of the opposite vertex (side $a$ is opposite angle $A$).
Which law to start with depends only on what you are given.
You know…
Name
Start with
two sides and the angle between them
SAS
Law of Cosines (third side)
all three sides
SSS
Law of Cosines (an angle)
two angles and any side
ASA or AAS
Law of Sines
two sides and an angle opposite one of them
SSA
Law of Sines (careful: see the ambiguous case)
Three angles (AAA) are not enough: they give the shape but not the size.
Practice★☆☆Not started
In a triangle $ABC$, the sides $a = 5$ in and $b = 8$ in are known, and the angle between them is $C = 60^\circ$. Calculate the length of the side $c$.
In a triangle $ABC$, the sides $a = 5$ in and $b = 8$ in are known, and the angle between them is $C = 60^\circ$. Calculate the length of the side $c$.
To solve a triangle means to find all three sides and all three angles. You usually need one law to get started and then the other (or the fact that the angles add up to $180^\circ$) to finish.
Practice★☆☆Not started
In a triangle $ABC$, the angles $A$ and $B$ measure $30^\circ$ and $105^\circ$ respectively. If the side opposite the angle $A$ has a length of $a = 8$ in, find the…
In a triangle $ABC$, the angles $A$ and $B$ measure $30^\circ$ and $105^\circ$ respectively. If the side opposite the angle $A$ has a length of $a = 8$ in, find the length of the side $c$ (opposite the angle $C$).
In an isosceles triangle, the unequal side (the base) measures $12$ ft and the equal angles measure $75^\circ$ each. Determine the length of the equal sides using the…
In an isosceles triangle, the unequal side (the base) measures $12$ ft and the equal angles measure $75^\circ$ each. Determine the length of the equal sides using the Law of Sines.
When you know two sides and an angle opposite one of them (SSA), there may be no triangle, one, or two. The reason: the side opposite the known angle can swing like a gate and hit the base line in two places.
Practice★☆☆Not started
In a triangle the following data are known: $a = 12$ ft, $b = 8$ ft and the angle $A = 120^\circ$. Calculate the measure of the angle $B$.
In a triangle the following data are known: $a = 12$ ft, $b = 8$ ft and the angle $A = 120^\circ$. Calculate the measure of the angle $B$.
In the design of an articulated robotic arm, the first link measures $40$ in and the second measures $35$ in. If the angle the first link forms with the horizontal base…
In the design of an articulated robotic arm, the first link measures $40$ in and the second measures $35$ in. If the angle the first link forms with the horizontal base is $50^\circ$ and the end of the second link exactly touches the horizontal base, calculate the two possible distances from the origin of the arm to the point of contact on the base.
You don't need the height to find an area. If you know two sides and the angle between them, there is a direct formula. If you know all three sides, there is Heron's formula.
Practice★☆☆Not started
Find the area of a triangle $ABC$ of which we know the sides $b = 14$ in, $c = 20$ in and the angle they form, $A = 60^\circ$.
Find the area of a triangle $ABC$ of which we know the sides $b = 14$ in, $c = 20$ in and the angle they form, $A = 60^\circ$.
In a triangle $ABC$, it is known that the side $a = 10$ in, the angle $B = 30^\circ$ and that the total area of the triangle is $20\sqrt{3}$ in$^2$. Set up a system to…
In a triangle $ABC$, it is known that the side $a = 10$ in, the angle $B = 30^\circ$ and that the total area of the triangle is $20\sqrt{3}$ in$^2$. Set up a system to find the exact value of the sides $b$ and $c$, and find out the measure of the angles $A$ and $C$.
Every triangle has a circumscribed circle (through all three vertices, radius $R$) and an inscribed circle (tangent to all three sides, radius $r$).
Practice★☆☆Not started
In a triangle $ABC$, the side $a$ measures $10$ in and the opposite angle $A$ measures $45^\circ$. Calculate the radius $R$ of the circumscribed circle of this triangle.
In a triangle $ABC$, the side $a$ measures $10$ in and the opposite angle $A$ measures $45^\circ$. Calculate the radius $R$ of the circumscribed circle of this triangle.
In a triangle $ABC$ with sides $a=13$ in., $b=14$ in., and $c=15$ in., the circumscribed circle of radius $R$ and the inscribed circle of radius $r$ are drawn. Calculate…
In a triangle $ABC$ with sides $a=13$ in., $b=14$ in., and $c=15$ in., the circumscribed circle of radius $R$ and the inscribed circle of radius $r$ are drawn. Calculate the area of the triangle using Heron's formula and, from it, obtain the exact values of $R$ and $r$.
Navigation and surveying problems often hide a triangle. The sides are distances, and the angles come from directions.
Clocks are another source of triangles: the minute hand moves $6^\circ$ per minute, and the hour hand moves $30^\circ$ per hour, which is $0.5^\circ$ per minute. The hour hand creeps; it is not exactly on the hour mark at 8:20.
Practice★★☆Not started
Two ships leave a port simultaneously on straight routes that form an angle of $75^\circ$ between them. The first ship sails at a constant speed of $15$ knots and the…
Two ships leave a port simultaneously on straight routes that form an angle of $75^\circ$ between them. The first ship sails at a constant speed of $15$ knots and the second at $20$ knots. Calculate the distance in nautical miles that will separate the two ships after $2$ hours of sailing.
The hands of a wall clock measure $12$ in. (the minute hand) and $8$ in. (the hour hand). Calculate the exact distance in inches separating the tips of both hands when…
The hands of a wall clock measure $12$ in. (the minute hand) and $8$ in. (the hour hand). Calculate the exact distance in inches separating the tips of both hands when the clock shows exactly 10:10. Take into account the proportional displacement of the hour hand.
Two airplanes fly at the same altitude. Airplane 1 is at point $A$ and flies toward the north at a constant speed of $800$ mph. Airplane 2 is at point $B$, located…
Two airplanes fly at the same altitude. Airplane 1 is at point $A$ and flies toward the north at a constant speed of $800$ mph. Airplane 2 is at point $B$, located exactly $200$ miles to the northeast ($45^\circ$ with respect to the north) of point $A$. If airplane 2 flies toward the west at a constant speed $v$ and both collide at a point $C$, determine the value of the speed $v$ and the time they take to collide.
To find the height of something you can't reach (a water tower, a wind turbine, a drone), measure angles from two spots on the ground. The two sight lines and the ground form a non-right triangle.
Practice★★☆Not started
To measure the height $h$ of a wind turbine, we stand at a point $A$ on the ground and measure the angle of elevation to the highest point, obtaining $40^\circ$. We…
To measure the height $h$ of a wind turbine, we stand at a point $A$ on the ground and measure the angle of elevation to the highest point, obtaining $40^\circ$. We advance $30$ feet in a straight line towards the base of the wind turbine until reaching a point $B$, from where the new angle of elevation is $55^\circ$. Calculate the height of the wind turbine.
A surveillance drone is located at a point $C$ observing a straight stretch of road delimited by two cities $A$ and $B$ that are $5$ miles apart. The angles of…
A surveillance drone is located at a point $C$ observing a straight stretch of road delimited by two cities $A$ and $B$ that are $5$ miles apart. The angles of depression from the drone to the cities are such that in the triangle formed, the angle at $A$ is $35^\circ$ and the angle at $B$ is $62^\circ$. Calculate the altitude at which the drone is flying with respect to the road.
A solar panel $2$ feet long is to be installed on a flat roof that has an inclination of $15^\circ$ with respect to the horizontal. To maximize its performance, the…
A solar panel $2$ feet long is to be installed on a flat roof that has an inclination of $15^\circ$ with respect to the horizontal. To maximize its performance, the panel must form an angle of $35^\circ$ with respect to the horizontal. Calculate the length $x$ of the vertical support that must be placed at the upper end of the panel to join it to the roof.
Any polygon can be split by diagonals into triangles. Solve each triangle with the laws above, then add the areas.
Parallelograms and regular polygons
In a parallelogram, adjacent angles add up to $180^\circ$. Two adjacent sides and their included angle give a diagonal by the Law of Cosines. The diagonal opposite the obtuse angle is the longer one.
For a regular polygon with $n$ sides inscribed in a circle of radius $R$, the center and two adjacent vertices form an isosceles triangle with apex angle $\dfrac{360^\circ}{n}$ and two sides of length $R$. Then: side $s^2 = 2R^2 - 2R^2\cos\dfrac{360^\circ}{n}$ and area $= n\cdot\tfrac12 R^2\sin\dfrac{360^\circ}{n}$.
Practice★☆☆Not started
The adjacent sides of a parallelogram measure $6$ in and $10$ in, and form an obtuse angle of $120^\circ$. Calculate the length of the longer diagonal of the…
The adjacent sides of a parallelogram measure $6$ in and $10$ in, and form an obtuse angle of $120^\circ$. Calculate the length of the longer diagonal of the parallelogram.
An agricultural plot of land has the shape of a quadrilateral $ABCD$. The measures of its sides are $AB = 50$ ft, $BC = 40$ ft, $CD = 30$ ft and $DA = 60$ ft. Knowing…
An agricultural plot of land has the shape of a quadrilateral $ABCD$. The measures of its sides are $AB = 50$ ft, $BC = 40$ ft, $CD = 30$ ft and $DA = 60$ ft. Knowing that the angle at the vertex $B$ is $70^\circ$, calculate the total area of the land by dividing it into two triangles.
A regular heptagon (polygon with $7$ equal sides) is inscribed in a circle of radius $10$ in. Using the Law of Cosines and/or Law of Sines in the central triangles,…
A regular heptagon (polygon with $7$ equal sides) is inscribed in a circle of radius $10$ in. Using the Law of Cosines and/or Law of Sines in the central triangles, calculate the perimeter and the area of this heptagon.
Two forces pulling on the same object add like vectors: the resultant is the diagonal of the parallelogram built on the two forces. That diagonal is the third side of a triangle, so the laws apply.
Practice★★☆Not started
Two tugboats pull a barge using two cables that form an angle of $45^\circ$ between them. If the tensions exerted by the cables are $4500$ N and $6000$ N, find the…
Two tugboats pull a barge using two cables that form an angle of $45^\circ$ between them. If the tensions exerted by the cables are $4500$ N and $6000$ N, find the magnitude of the resultant force and the angle this resultant forms with the cable exerting the force of $6000$ N.
Sometimes a side is an algebraic expression. Then the Law of Cosines becomes an equation you solve with Integrated Math III tools. Always check that your answer makes the lengths positive.
The projection formula
In any triangle $ABC$, $a = b\cos C + c\cos B$. The Law of Cosines also lets us prove the sum formula for sine.
Practice★★★Not started
The sides of a triangle are given by the expressions $x$, $x+2$ and $x+4$ inches. Knowing that the largest angle of the triangle measures $120^\circ$, set up the…
The sides of a triangle are given by the expressions $x$, $x+2$ and $x+4$ inches. Knowing that the largest angle of the triangle measures $120^\circ$, set up the corresponding equation, find the exact value of $x$ and calculate the perimeter of the triangle.
Prove geometrically or by using trigonometric laws (Law of Sines and Law of Cosines) that in any triangle $ABC$ the projection relation holds: $a = b \cos C + c \cos B$.…
Prove geometrically or by using trigonometric laws (Law of Sines and Law of Cosines) that in any triangle $ABC$ the projection relation holds: $a = b \cos C + c \cos B$. Then, use this identity together with the Law of Sines to prove the formula for the sine of the sum of two angles: $\sin(B+C) = \sin B \cos C + \cos B \sin C$.
Two challenge techniques show how powerful the laws are: finding the length of an angle bisector, and flattening a 3-D figure into a triangle.
Length of an angle bisector
A pyramid's dihedral angle
The dihedral angle between two faces is the angle between them measured in a plane perpendicular to their common edge. To use it, find that plane and look for a triangle.
Practice★★★Not started
Given triangle $ABC$ with sides $a = 7$ in., $b = 5$ in., and $c = 8$ in. The interior angle bisector of angle $C$ is drawn, which intersects side $AB$ at point $D$.…
Given triangle $ABC$ with sides $a = 7$ in., $b = 5$ in., and $c = 8$ in. The interior angle bisector of angle $C$ is drawn, which intersects side $AB$ at point $D$. Applying the Law of Cosines to find the angle $C$ and using the properties of areas or the Law of Sines, calculate the exact length of the bisector segment $CD$ (denoted as $x$).
A regular square-based pyramid has a base edge of $100$ ft. Knowing that the dihedral angle formed by two adjacent triangular faces is $105^\circ$, set up the geometry…
A regular square-based pyramid has a base edge of $100$ ft. Knowing that the dihedral angle formed by two adjacent triangular faces is $105^\circ$, set up the geometry of the appropriate cross-section and calculate the total height of the pyramid.
Back to the start: Emma and Marcus have two sides (3 mi and 4 mi) and the angle between them ($70^\circ$), which is SAS, so $d^2 = 3^2 + 4^2 - 2\cdot 3\cdot 4\cos 70^\circ \approx 16.79$ and $d \approx 4.1$ mi.
Summary
Let's practice! 8 medium exercises in this section