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Secondary 6 · Mathematics
Equations · Systems of equations

Nonlinear Systems

Wilkin wants to fence off a rectangular lot. He knows the perimeter is 40 m and the area is 96 m². How long are the sides? One of the equations has a product of unknowns: it is no longer linear. In this section you will learn to solve systems like this, using the same idea you used with linear systems: solve for a variable in the simplest equation and substitute.

What a nonlinear system is

Comparison between a linear equation that is easy to solve for a variable and a nonlinear equation with a product of unknowns. x − y = 4(linear, easy to solve for a variable) x · y = 12(nonlinear)

Graphical interpretation

Each equation is a curve in the plane. The solutions of the system are the points of intersection of the curves. A line and a parabola can intersect at 0, 1, or 2 points; the same is true for a line and a circle.

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 intersections2 solutions tangent: one intersection1 solution no intersection0 solutions

Try the hyperbola $y = 12/x$ and the line $y = x - 4$, and observe where they intersect:

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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

Problems that lead to nonlinear systems

Many problems involving areas, perimeters, volumes, sharing, or money end up as a system with a product or a square.

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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 $7\,200$ 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 $7\,200$ pesos, splitting the cost equally. At the last minute, two more friends join the group, which reduces each person's share by $180$ 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 $660\,000$ 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 $660\,000$ pesos. At the end of the second year, the amount rises to $726\,000$ pesos. Set up a non-linear system to calculate the initial principal $C$ and the interest rate $r$, and solve it.

Solve

Symmetric systems

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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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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

Algebraic fractions in the system

When fractions appear, clear the denominators (with $x \neq 0$, $y \neq 0$, etc.) or use an identity that relates the unknowns.

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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

Exponential and logarithmic systems

Diagram showing how setting the exponents equal in powers with the same base turns an exponential equation into a linear one. 2x+y = 25 2x−y = 21 same base → set exponents equal x + y = 5 x − y = 1 linear system
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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
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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

Systems with radicals

Solution process with a radical: squaring introduces a false solution that is discarded when checking. √x = 6 − x x² − 13x + 36 = 0 x = 4 ó x = 9 x = 4 ✓satisfies √x = 6−x x = 9 ✗does not satisfy
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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

Higher-degree symmetric systems

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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

Discussing with a parameter: tangency

When the system has a parameter, the number of real solutions is studied with the discriminant $\Delta$ of the quadratic equation that results from substituting: $\Delta > 0$ gives two solutions, $\Delta = 0$ gives one (tangency), and $\Delta < 0$ gives none.

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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

Summary

See all the exercises in this section