Question

Why do you replace y with 0 when finding the x-intercept of a graph?

Answer 1

Here’s what you need to know about intercepts. The x intercept of a graph has y coordinate zero. The y intercept of a graph has x coordinate zero.

AN EXAMPLE OF THIS IS:

So if you have a function y = 2x + 8 you would set x = 0 to find the y intercept. So we would get y = 8. To find the x intercept we would set y = 0 and get 0 = 2x + 8

or -8 = 2x or x = -4

Now if you have a more difficult like function y = 3x^2 + 10x - 4 we proceed the same way. To find the y intercept we would set x = 0 and get y = -4 so the y intercept is (0,-4)

To find the x intercept we would set y = 0 and get 3x^2 + 10x - 4 = 0. If you know the quadratic formula you could solve this by:

x = [-b +- SR(b^2 - 4*a*c] / 2a

so in our case x = [-10 +- SR(100 - 4*3*(-4))] / (2*3)

x = [-10 +- SR(100 + 48)] / 6

x = [-10 +- SR (148)] / 6

x = [-10 +- 12.2] / 6 approximately

x = [-10 - 12.2] / 6 or [-10 + 12.2 / 6]

x = -22.2/6 or 2.2/6