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Equations · Systems of equations

Nonlinear systems

Before you start, make sure you know: Systems of nonlinear equations (High School Year 1 (semesters 1 and 2))
Before you start, make sure you know: Logarithms (High School Year 1 (semesters 1 and 2))

A rectangular plot of land in Oaxaca has a perimeter of $28$ m and an area of $48$ m². How long are its sides? One equation is not enough: there are two unknowns and one of the equations has a product. At the end of the page we solve it.

What a nonlinear system is

The usual method still works: isolate one unknown in the simplest equation and substitute it into the other. It almost always ends in a quadratic equation. Geometrically, the solutions are the points where the two graphs cross.

The parabola $y=x^2-4x+3$ and the line $y=x-1$ intersect at two points: the system has two solutions.

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Solve the following non-linear system, which represents the intersection between a parabola and a line: $y = x^2 - 4x + 3$ $y = x - 1$

Solve the following non-linear system, which represents the intersection between a parabola and a line:

$y = x^2 - 4x + 3$

$y = x - 1$

Solve
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Find the solutions to the following system formed by the equation of a circle and a line: $x^2 + y^2 = 25$ $x + y = 7$

Find the solutions to the following system formed by the equation of a circle and a line:

$x^2 + y^2 = 25$

$x + y = 7$

Solve
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Find the values of $x$ and $y$ that satisfy the following system: $xy = 12$ $x - y = 4$

Find the values of $x$ and $y$ that satisfy the following system:

$xy = 12$

$x - y = 4$

Solve

Exponential and logarithmic systems

By writing both powers with the same base, the exponents are equated and the exponential system becomes linear. 2^(x+y) = 2^5 2^(x−y) = 2^1 → x + y = 5 x − y = 1

When there are sums of powers

If $3^x + 2^y$ appears, you cannot equate exponents. You make a change of variable: $u=3^x$, $v=2^y$. Notice that $3^{x+1}=3\cdot 3^x=3u$ and $2^{y-1}=\frac{2^y}{2}=\frac{v}{2}$.

A power is always positive: if, when solving the linear system, $u$ or $v$ come out negative or zero, the system has no solution.

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Solve the following system of exponential equations by expressing both powers with the same base: $2^{x+y} = 32$ $2^{x-y} = 2$

Solve the following system of exponential equations by expressing both powers with the same base:

$2^{x+y} = 32$

$2^{x-y} = 2$

Solve
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Determine the solution to the logarithmic system by applying the laws of logarithms: $\log x + \log y = 1$ $x - y = 3$

Determine the solution to the logarithmic system by applying the laws of logarithms:

$\log x + \log y = 1$

$x - y = 3$

Solve
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Solve the following system of exponential equations by applying a suitable change of variable: $3^x + 2^y = 11$ $3^{x+1} - 2^{y-1} = 26$

Solve the following system of exponential equations by applying a suitable change of variable:

$3^x + 2^y = 11$

$3^{x+1} - 2^{y-1} = 26$

Solve
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Solve the logarithmic and quadratic system, checking the validity of the solution obtained: $\log(x) - \log(y) = 1$ $x^2 - y^2 = 99$

Solve the logarithmic and quadratic system, checking the validity of the solution obtained:

$\log(x) - \log(y) = 1$

$x^2 - y^2 = 99$

Solve

Systems with radicals

Squaring an equation with a root creates a false solution that is discarded by checking in the original equation. √x = 6 − x → x = 4 ó x = 9 → x = 4 ✓ x = 9 ✗

To remove a root, you square both sides. It is best to leave the root alone on one side before squaring.

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Solve the following radical system. Make sure to check whether the solution obtained is valid in the original equation: $\sqrt{x} + y = 5$ $x - y = 1$

Solve the following radical system. Make sure to check whether the solution obtained is valid in the original equation:

$\sqrt{x} + y = 5$

$x - y = 1$

Solve
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Find the solution to the following radical system: $\sqrt{x+y} + \sqrt{x-y} = 4$ $x - y = 4$

Find the solution to the following radical system:

$\sqrt{x+y} + \sqrt{x-y} = 4$

$x - y = 4$

Solve
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Solve the following radical system, making sure to check which solutions are valid by analysing the domain of the roots: $\sqrt{x^2 - y^2} = \sqrt{33}$ $x + y = 11$

Solve the following radical system, making sure to check which solutions are valid by analysing the domain of the roots:

$\sqrt{x^2 - y^2} = \sqrt{33}$

$x + y = 11$

Solve

Sum and product, fractions, and higher degree

Key idea: if $x+y=S$ and $xy=P$, then $x$ and $y$ are the roots of $t^2-St+P=0$.

A rectangle with sides x and y: its perimeter gives the sum x plus y and its area gives the product x times y. x y area = x · y perimeter = 2(x + y)

With that rectangle we close the opening problem: $2(x+y)=28$ gives $x+y=14$ and the area gives $xy=48$.

Fractions, and a circle with a hyperbola

If $\frac1x+\frac1y$ appears, add them: $\frac{x+y}{xy}$. With $xy=6$ and $\frac1x+\frac1y=\frac56$ we get $\frac{x+y}{6}=\frac56$, that is $x+y=5$: sum and product again.

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Solve the following system containing algebraic fractions: $\frac{1}{x} + \frac{1}{y} = \frac{5}{6}$ $xy = 6$

Solve the following system containing algebraic fractions:

$\frac{1}{x} + \frac{1}{y} = \frac{5}{6}$

$xy = 6$

Solve
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The sum of two positive numbers is $10$ and the sum of their squares is $58$. Set up a non-linear system of equations and solve it to find these numbers.

The sum of two positive numbers is $10$ and the sum of their squares is $58$. Set up a non-linear system of equations and solve it to find these numbers.

Solve
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Solve the following system formed by a rational equation and a quadratic equation: $\frac{x+y}{x-y} = 5$ $x^2 + y^2 = 13$

Solve the following system formed by a rational equation and a quadratic equation:

$\frac{x+y}{x-y} = 5$

$x^2 + y^2 = 13$

Solve
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The circular orbit of a satellite in a reference frame follows the equation $x^2 + y^2 = 25$, while the path of a comet describes the hyperbola $xy = 12$. Solve the…

The circular orbit of a satellite in a reference frame follows the equation $x^2 + y^2 = 25$, while the path of a comet describes the hyperbola $xy = 12$. Solve the system to find the exact coordinates of the intersection points of both paths.

X Y x² + y² = 25 xy = 12
Solve
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Solve the higher-degree system of equations. (Hint: use algebraic identities and factorisation to simplify the first equation): $x^3 + y^3 = 35$ $x + y = 5$

Solve the higher-degree system of equations. (Hint: use algebraic identities and factorisation to simplify the first equation):

$x^3 + y^3 = 35$

$x + y = 5$

Solve

Setting up word problems

To set up a problem: name the unknowns, write one equation for each condition, solve, and check that the solution makes sense (lengths and numbers of people are positive). Amounts are given in pesos.

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A rectangular plot has a perimeter of $40$ m and an area of $96$ m$^2$. Set up a non-linear system of equations to calculate the dimensions of the plot and solve it.

A rectangular plot has a perimeter of $40$ m and an area of $96$ m$^2$. Set up a non-linear system of equations to calculate the dimensions of the plot and solve it.

x y A = 96 m² P = 40 m Length
Solve
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A cyclist completes a $60$ km journey at a constant speed. If they had travelled $2$ km/h faster, it would have taken them $1$ hour less to complete the journey. Set up…

A cyclist completes a $60$ km journey at a constant speed. If they had travelled $2$ km/h faster, it would have taken them $1$ hour less to complete the journey. Set up and solve a non-linear system to find the average speed $v$ and the time $t$ taken.

Solve
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A group of friends decides to buy a birthday present costing $2\,400$ pesos, splitting the cost equally. At the last minute, two more friends join the group, which…

A group of friends decides to buy a birthday present costing $2\,400$ pesos, splitting the cost equally. At the last minute, two more friends join the group, which reduces each person's share by $60$ pesos. Set up and solve a system to find how many friends there were initially and how much each person paid.

Solve
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In a right-angled triangle, the hypotenuse measures $13$ cm and the total perimeter is $30$ cm. Set up a system of equations to determine the lengths of the two legs.

In a right-angled triangle, the hypotenuse measures $13$ cm and the total perimeter is $30$ cm. Set up a system of equations to determine the lengths of the two legs.

x y 13 cm P = 30 cm
Solve
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A sheet-metal cylindrical tank has an interior volume of $54\pi$ m³ and a curved surface area (excluding the bases) of $36\pi$ m². Set up a system to find the base…

A sheet-metal cylindrical tank has an interior volume of $54\pi$ m³ and a curved surface area (excluding the bases) of $36\pi$ m². Set up a system to find the base radius $r$ and the height $h$ of the cylinder, and solve it.

r h V = 54π Aₗ = 36π
Solve
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A principal $C$ is invested at an annual compound interest rate $r$ (expressed as a decimal). At the end of the first year, the accumulated amount is $220\,000$ pesos.…

A principal $C$ is invested at an annual compound interest rate $r$ (expressed as a decimal). At the end of the first year, the accumulated amount is $220\,000$ pesos. At the end of the second year, the amount rises to $242\,000$ pesos. Set up a non-linear system to calculate the initial principal $C$ and the interest rate $r$, and solve it.

Solve

More changes of variable

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Solve the following mixed system combining exponential and logarithmic equations: $2^x \times 2^y = 64$ $\log_2 x + \log_2 y = 3$

Solve the following mixed system combining exponential and logarithmic equations:

$2^x \times 2^y = 64$

$\log_2 x + \log_2 y = 3$

Solve
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Solve the following logarithmic system by applying change-of-base properties and substitution: $\log_y x + \log_x y = \frac{5}{2}$ $xy = 27$

Solve the following logarithmic system by applying change-of-base properties and substitution:

$\log_y x + \log_x y = \frac{5}{2}$

$xy = 27$

Solve

Discussing with a parameter: tangency

A line can cut a curve at two points, touch it at one (tangent), or not touch it at all. With a parameter, the number of solutions is studied with the discriminant $\Delta=b^2-4ac$ of the resulting quadratic.

Three positions of a line relative to a circle: secant with two intersections, tangent with a single point, and exterior with no common points. two intersectionsΔ > 0 tangentΔ = 0 no intersectionΔ < 0
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Discuss for which values of the parameter $k$ the following system has exactly one solution (geometrically, this corresponds to the tangency condition between the circle…

Discuss for which values of the parameter $k$ the following system has exactly one solution (geometrically, this corresponds to the tangency condition between the circle and the line):

$x^2 + y^2 = 10$

$x + y = k$

Solve

Back to the beginning: the plot measures $6$ m by $8$ m. With the sum ($14$) and the product ($48$) we set up $t^2-14t+48=0$.

Summary

See all the exercises in this section