Nonlinear systems
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.
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:
$y = x^2 - 4x + 3$
$y = x - 1$
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:
$x^2 + y^2 = 25$
$x + y = 7$
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:
$xy = 12$
$x - y = 4$
Exponential and logarithmic systems
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.
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:
$2^{x+y} = 32$
$2^{x-y} = 2$
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 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 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:
$\log(x) - \log(y) = 1$
$x^2 - y^2 = 99$
Systems with radicals
To remove a root, you square both sides. It is best to leave the root alone on one side before squaring.
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:
$\sqrt{x} + y = 5$
$x - y = 1$
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 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:
$\sqrt{x^2 - y^2} = \sqrt{33}$
$x + y = 11$
Sum and product, fractions, and higher degree
Key idea: if $x+y=S$ and $xy=P$, then $x$ and $y$ are the roots of $t^2-St+P=0$.
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.
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:
$\frac{1}{x} + \frac{1}{y} = \frac{5}{6}$
$xy = 6$
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 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:
$\frac{x+y}{x-y} = 5$
$x^2 + y^2 = 13$
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.
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):
$x^3 + y^3 = 35$
$x + y = 5$
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.
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.
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.
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.
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.
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.
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.
More changes of variable
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 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:
$\log_y x + \log_x y = \frac{5}{2}$
$xy = 27$
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.
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):
$x^2 + y^2 = 10$
$x + y = k$
Back to the beginning: the plot measures $6$ m by $8$ m. With the sum ($14$) and the product ($48$) we set up $t^2-14t+48=0$.