Answer:
1. Look at the screenshot below.
2. If a line intersects a parabola at a point, the coordinates of the intersection point must satisfy the equation of the line and the equation of the parabola.
Since the equation of the line is y = c, where c is a constant, the y-coordinate of the intersection point must be c.
It follows then that substituting c for y in the equation for the parabola will result in another true equation: c = −x^2 + 5x.
Subtracting c from both sides of c = −x^2 + 5x and then dividing both sides by −1 yields 0 = x^2 − 5x + c.
The solution to this quadratic equation would give the x-coordinate(s) of the point(s) of intersection.
Since it’s given that the line and parabola intersect at exactly one point, the equation 0 = x^2 − 5x + c has exactly one solution.
A quadratic equation in the form 0 = ax^2 + bx + c has exactly one solution when its discriminant b 2 − 4ac is equal to 0. In the equation 0 = x^2 − 5x + c, a = 1, b = −5, and c = c.
Therefore, (−5)^2 − 4(1)(c ) = 0, or 25 − 4c = 0.
Subtracting 25 from both sides of 25 − 4c = 0 and then dividing both sides by −4 yields c = 25/4 .
Therefore, if the line y = c intersects the parabola defined by y = −x^2 + 5x at exactly one point, then c = 25/4 .
Either 25/4 or 6.25
