Systems of Polynomial Equations

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PSAT Math › Systems of Polynomial Equations

Questions 1 - 10
1

A designer models a ramp by the line $y=\tfrac{1}{2}x+1$ and an arch above it by the parabola $y=-\tfrac{1}{2}x^2+2x+1$. The ramp meets the arch at the intersection points. What is the product of the $x$-coordinates of the intersection points?

$0$

$2$

$8$

$4$

Explanation

We need the product of x-coordinates where the line y = ½x + 1 meets the parabola y = -½x² + 2x + 1. This system combines a line and parabola, solved by substitution. Setting ½x + 1 = -½x² + 2x + 1, we get ½x + 1 + ½x² - 2x - 1 = 0, which simplifies to ½x² - (3/2)x = 0, or ½x(x - 3) = 0. This gives x = 0 or x = 3, so the product of x-coordinates is 0 × 3 = 0. A key insight: when one root is zero, the product of all roots must be zero. For any quadratic ax² + bx + c = 0, Vieta's formula gives the product of roots as c/a, and here c = 0 confirms our answer.

2

On the coordinate plane, the graphs of the line $y=x+1$ and the parabola $y=x^2-1$ are shown. How many solutions does the system have (how many intersection points are there)?

0

1

2

3

Explanation

The question asks for the number of solutions to the system, which corresponds to the intersection points of the line and parabola. This is a system of one linear and one nonlinear equation, solved by substitution, emphasizing that a parabola and line can intersect 0, 1, or 2 times. Set x + 1 = x² - 1, rearrange to x² - x - 2 = 0, and factor as (x - 2)(x + 1) = 0, giving x = 2 and x = -1. These yield two distinct real solutions. A key error might be miscounting roots if the discriminant is overlooked, but here it's positive with two roots. A strategy is to calculate the discriminant quickly to determine the number of intersections without full solving.

3

A designer models the edge of a ramp with $y=x^2-4x+3$ and a support beam with $y=x-1$. At what points do the graphs intersect? Solve the system algebraically.

$(0,-1)$ and $(3,2)$

$(1,0)$ and $(4,3)$

$(2,1)$ and $(3,2)$

$(1,0)$ only

Explanation

The question seeks the intersection points of the quadratic ramp model and linear support beam. This system combines a linear and a nonlinear quadratic equation, solved via substitution, with the reminder that such graphs can intersect at 0, 1, or 2 points. Set x² - 4x + 3 = x - 1, simplify to x² - 5x + 4 = 0, and factor as (x - 1)(x - 4) = 0, so x = 1 and x = 4. Corresponding y-values are y = 0 for x = 1 and y = 3 for x = 4, giving (1, 0) and (4, 3). Errors often occur in algebraic simplification, like incorrect subtraction. When time is limited, graph mentally or check discriminants to confirm two real roots.

4

On the coordinate plane, the line $y=x+2$ and the parabola $y=x^2$ are graphed. How many solutions does the system have (that is, how many intersection points are there)?​​

0

1

2

3

Explanation

The question asks for the number of solutions to the system of y = x + 2 and y = x², which represents the intersection points of a line and a parabola on the coordinate plane. This system includes one linear and one nonlinear equation, solved by substitution: set x² = x + 2, rearrange to x² - x - 2 = 0, and factor as (x - 2)(x + 1) = 0, giving x = 2 and x = -1. These yield two distinct real solutions, confirming two intersection points. A key error could be incorrectly factoring the quadratic or overlooking the discriminant (here, 1 + 8 = 9 > 0, indicating two real roots). Note that a parabola and a line can intersect 0, 1, or 2 times, and in this case, it's two; a useful strategy is to calculate the discriminant of the resulting quadratic to quickly determine the number of solutions without fully solving.

5

A student solves the system $y=x^2+2x$ and $y=4x$ to find when a quadratic growth model matches a linear estimate. What is the product $x_1x_2$ of the $x$-values of all solutions?

$0$

$8$

$-8$

$-4$

Explanation

The question seeks the product x₁x₂ of x-values where quadratic and linear models match. This is a linear-nonlinear system, solved by substitution, with up to 2 intersections possible for line and parabola. Set x² + 2x = 4x, simplify to x² - 2x = 0, factor as x(x - 2) = 0, so x = 0 and x = 2, product 0. For ax² + bx + c = 0, product is c/a, here 0/1 = 0. A common error is ignoring the x=0 solution. Use quadratic properties like product of roots to answer without listing roots.

6

Solve the system algebraically: $$y=x^2-4x+1$$ and $$y=-x+3.$$ How many real solutions does the system have?

0

1

2

3

Explanation

We're asked to count the number of real solutions when y = x² - 4x + 1 intersects y = -x + 3. This system combines a parabola with a line, solved by substitution. Setting x² - 4x + 1 = -x + 3, we get x² - 4x + 1 + x - 3 = 0, which simplifies to x² - 3x - 2 = 0. Using the quadratic formula or factoring, we find the discriminant is (-3)² - 4(1)(-2) = 9 + 8 = 17 > 0, indicating two distinct real solutions. The key insight is that a positive discriminant means the line crosses the parabola at two points. When solving systems graphically or algebraically, remember that the number of solutions equals the number of intersection points.

7

A drone’s height is modeled by $y=x^2-6x+8$, while a safety limit is $y=0$. The drone is at the safety limit when the graphs intersect. How many solutions does the system have?$$\begin{cases}y=x^2-6x+8\\y=0\end{cases}$$

0

1

2

4

Explanation

The question asks how many solutions the system has, meaning how many times the drone's height model intersects the safety limit y=0. This involves setting a quadratic equal to a constant (horizontal line), solvable by finding roots of the quadratic equation, with 0, 1, or 2 possible intersections. Set $x^2 - 6x + 8 = 0$. Factor as $(x - 2)(x - 4) = 0$, giving $x = 2$ and $x = 4$, so two solutions. A common error is misfactoring or ignoring the constant term. A test-taking strategy is to compute the discriminant $(36 - 32 = 4 > 0)$ to confirm two real roots quickly.

8

A ball’s height $y$ (in meters) after $x$ seconds is modeled by $y=-x^2+6x+1$. A timing sensor also records the height as $y=2x+1$ over the same interval. At what point(s) $(x,y)$ do these two models agree (i.e., where do the graphs intersect)? Solve the system algebraically.​​

$(1,3)$ and $(5,11)$

$(1,3)$ only

$(0,1)$ and $(4,9)$

$(0,1)$ only

Explanation

The question asks for the points where two models of a ball's height agree, meaning the intersection points of the quadratic equation y = -x² + 6x + 1 and the linear equation y = 2x + 1. This is a system consisting of one linear and one nonlinear (quadratic) equation, which can be solved algebraically by setting the equations equal to each other. Set -x² + 6x + 1 = 2x + 1, subtract 2x + 1 from both sides to get -x² + 4x = 0, then multiply by -1 to obtain x² - 4x = 0, and factor as x(x - 4) = 0, yielding x = 0 or x = 4. Substitute these x-values back into y = 2x + 1 to find y = 1 when x = 0 and y = 9 when x = 4, so the points are (0, 1) and (4, 9). A common error is mishandling the signs during subtraction, which might lead to an incorrect quadratic equation. Remember that a line and a parabola can intersect at 0, 1, or 2 points, and here they intersect at two; for test-taking, verify solutions by plugging back into both original equations.

9

A ball’s height (in meters) after $t$ seconds is modeled by $h=-t^2+6t+1$. A nearby sensor reports the height is also given by the line $h=2t+1$. The ball and the sensor agree at the times when the graphs intersect. What are all solutions $(t,h)$ to the system?

$(1,3)$ and $(3,7)$

$(1,3)$ only

$(0,1)$ and $(2,5)$

$(0,1)$ and $(4,9)$

Explanation

We need to find where the ball's height h = -t² + 6t + 1 equals the sensor's reading h = 2t + 1. This is a system with a parabola (quadratic) and a line, which we solve by substitution. Setting -t² + 6t + 1 = 2t + 1, we get -t² + 6t + 1 - 2t - 1 = 0, which simplifies to -t² + 4t = 0, or -t(t - 4) = 0. This gives t = 0 or t = 4. When t = 0, h = 2(0) + 1 = 1, and when t = 4, h = 2(4) + 1 = 9, so our solutions are (0,1) and (4,9). A common error is forgetting to find the h-values after solving for t. Since we found two distinct intersection points, this confirms that a parabola and line can intersect at 0, 1, or 2 points.

10

A coach tracks a runner’s distance $y$ with two models: a curved estimate $y=x^2-6x+8$ and a linear estimate $y=-2x+4$, where $x$ is time. If $(x_1,y_1)$ and $(x_2,y_2)$ are intersection points, what is $x_1x_2$?

$$\begin{cases} y=x^2-6x+8 \\ y=-2x+4 \end{cases}$$

$4$

$0$

$-4$

$8$

Explanation

We need to find the product of x-coordinates where $y = x^2 - 6x + 8$ and $y = -2x + 4$ intersect. This quadratic-linear system uses substitution. Setting equal: $x^2 - 6x + 8 = -2x + 4$, which becomes $x^2 - 4x + 4 = 0$. This factors as $(x - 2)^2 = 0$, giving $x = 2$ as a repeated root. With only one x-value (multiplicity 2), we have $x_1 = x_2 = 2$, so $x_1 x_2 = 2 imes 2 = 4$. A common error is thinking one solution means the product is undefined, but a repeated root counts twice. For any quadratic $ax^2 + bx + c = 0$, the product of roots equals $c/a$, which here is $4/1 = 4$.

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