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Math III · Mathematics
Equations · Systems of Equations

Non-Linear Systems of Equations

During an NFL game, a camera hangs from a cable stretched in a straight line above the field. A punt flies through the air along a parabola. Does the ball ever touch the cable, and if so, where? Finding the crossing points means solving a system in which one equation is not linear. By the end of this page you will know how to answer that question, and how to handle circles, hyperbolas, exponentials, logs and radicals too.

What is a non-linear system?

The main method is the same one you used in Integrated Math I: isolate a variable in the simplest equation and substitute into the other one. If both equations are already solved for $y$, just set the two expressions equal.

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

Find the solutions of 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

Sum and product

Some systems tell you the sum and the product of two numbers, or something that leads to them. These are called symmetric systems: swapping $x$ and $y$ leaves the system unchanged.

A rectangle with sides x and y whose perimeter gives the sum x + y = 14 and whose area gives the product xy = 45. Area = xy = 45 x y Perimeter 28, so x + y = 14 x · y = 45 t² − 14t + 45 = 0 x and y are the roots
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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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A rectangular plot has a perimeter of $40$ ft and an area of $96$ sq ft$^2$. 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 $40$ ft and an area of $96$ sq ft$^2$. Set up a non-linear system of equations to calculate the dimensions of the plot and solve it.

x y A = 96 sq ft P = 40 ft Length
Solve
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In a right triangle, the hypotenuse measures $13$ inches and the total perimeter is $30$ inches. Set up a system of equations to determine the length of the two legs.

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

x y 13 in P = 30 in
Solve

Systems with fractions

When the unknowns sit in denominators, rewrite the fractions so the system turns into something you already know how to solve.

The sum of reciprocals 1/x + 1/y written over a common denominator as (x + y)/(xy), which is sum over product. 1/x + 1/y = (x + y)/(xy) Common denominator: sum over product
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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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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

Using identities: squares and cubes

Algebraic identities let you turn a hard equation into a sum-and-product problem.

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Solve the system of higher degree equations. (Hint: use algebraic identities and factor to simplify the first equation): $x^3 + y^3 = 35$ $x + y = 5$

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

$x^3 + y^3 = 35$

$x + y = 5$

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

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

X Y x² + y² = 25 xy = 12
Solve

Exponential systems

Writing both sides as powers of 5 and setting the exponents equal turns an exponential system into a linear system. 5x+y = 53 5x−y = 51 same base, so exponents are equal x + y = 3 x − y = 1 linear system
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Solve the following system of exponential equations, expressing both powers with the same base: $2^{x+y} = 32$ $2^{x-y} = 2$

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

$2^{x+y} = 32$

$2^{x-y} = 2$

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

Logarithmic systems

Three logarithm rules used in systems: product rule, quotient rule, and the link between logarithms and exponents. log a + log b = log(ab) log a − log b = log(a/b) log₂ 8 = 3 means 2³ = 8 Arguments must be positive: a > 0 and b > 0

In this course $\log x$ means $\log_{10} x$. Use the product and quotient rules to combine logs into one, then rewrite as an exponential equation. Always check the domain at the end.

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Determine the solution of the logarithmic system, applying the properties of logarithms: $\log x + \log y = 1$ $x - y = 3$

Determine the solution of the logarithmic system, applying the properties of logarithms:

$\log x + \log y = 1$

$x - y = 3$

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

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

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

$x^2 - y^2 = 99$

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

Solve the following mixed system combining exponential and logarithmic equations:

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

$\log_2 x + \log_2 y = 3$

Solve

A hidden quadratic: logs with different bases

Sometimes the same expression appears twice, once flipped. Name it with a new variable and the system reveals a quadratic.

Two logarithms with swapped base and argument are reciprocals: if one is u, the other is 1/u. logy x = u logx y = 1/u Swapping base and argument gives the reciprocal
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Solve the following logarithmic system by applying the properties of change of base and substitution: $\log_y x + \log_x y = \frac{5}{2}$ $xy = 27$

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

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

$xy = 27$

Solve

Systems with radicals

Squaring a radical equation produces two candidate solutions, but checking in the original equation shows that only one works. √x = 12 − x x² − 25x + 144 = 0 x = 9 or x = 16 x = 9 ✓3 = 12 − 9 works x = 16 ✗4 ≠ −4, rejected
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Solve the following radical system. Make sure to check if the obtained solution is valid in the original equation: $\sqrt{x} + y = 5$ $x - y = 1$

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

$\sqrt{x} + y = 5$

$x - y = 1$

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

Find the solution of 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 analyzing the domain of the radicals: $\sqrt{x^2 - y^2} = \sqrt{33}$ $x + y = 11$

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

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

$x + y = 11$

Solve

Word problems: rates and sharing

Four steps for modeling a word problem with a non-linear system: name the unknowns, write two equations, substitute and solve, check in the story. 1. Name theunknowns 2. Write twoequations 3. Substituteand solve 4. Check inthe story

Many "quantity = rate × amount" stories give a product equation. If a change in one factor changes the other, you get a second product equation, and the pair is a non-linear system.

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A cyclist completes a journey of $60$ miles at a constant speed. If they had traveled $2$ mph faster, they would have taken $1$ hour less to complete the route. Set up…

A cyclist completes a journey of $60$ miles at a constant speed. If they had traveled $2$ mph faster, they would have taken $1$ hour less to complete the route. 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 that costs $\$120…

A group of friends decides to buy a birthday present that costs $\$120$, dividing the payment equally. At the last minute, two more friends join the group, which reduces each person's share by $\$3$. Set up and solve a system to find how many friends there were initially and how much each paid.

Solve

Dividing equations: geometry and growth

When both equations are products of the same unknowns, dividing one equation by the other can cancel unknowns right away.

A cylinder with base radius r and height h, with its volume and lateral area formulas. r = radius of the base h V = πr²h L = 2πrh (lateral area)
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A cylindrical sheet metal tank has an internal volume of $54\pi$ cubic feet and a lateral area (excluding the bases) of $36\pi$ square feet. Set up a system to find the…

A cylindrical sheet metal tank has an internal volume of $54\pi$ cubic feet and a lateral area (excluding the bases) of $36\pi$ square feet. 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 amount generated is $\$11000…

A principal $C$ is invested at an annual compound interest rate $r$ (expressed as a decimal). At the end of the first year, the amount generated is $\$11000$. At the end of the second year, the amount rises to $\$12100$. Set up a non-linear system to calculate the initial principal $C$ and the interest rate $r$, and solve it.

Solve

How many solutions? Tangency

A line can cut a curve in two points, touch it in exactly one point (it is tangent), or miss it completely. When the system has a parameter, substituting leaves a quadratic whose discriminant $\Delta = b^2 - 4ac$ decides how many real solutions there are.

Three positions of a line relative to a circle: secant with two intersection points, tangent with one, and external with none. cuts twice2 solutions tangent: one point1 solution misses the circle0 solutions

Back to the football. Set the parabola equal to the cable's line. You get a quadratic. If $\Delta > 0$, the ball crosses the cable twice (going up and coming down). If $\Delta = 0$, it just grazes the cable. If $\Delta < 0$, the ball never reaches it.

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Discuss for which values of the parameter $k$ the following system has exactly one solution (this is geometrically equivalent to the condition of tangency between the…

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

$x^2 + y^2 = 10$

$x + y = k$

Solve

Summary

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