Question

When graphing y = 2x2 + 35x + 75, which viewing window would allow you to see all of the intercepts and the minimum as closely as possible?

Answer 1

The intercept with the y-axes occurs when x = 0

f(0) = 2*02 + 35*0 + 75 = 0 + 0 + 75 = 75

So intercept with x axes is the point (0,75)

The intercept with the x-axes occurs when y=0

2x2 + 35x + 75 = 0

As there’s no direct way to find x, we can use the Bhaskara formula:

x=(-b+- square root (b2-4ac))/(2a)

This formula is for: ax2 +bx + c = 0

So:

a=2

b=35

c=75

x=(-35+- square root (35^2-4*2*75))/(2*2)

So x can be -2.5 or -15

So intercept with y axes are the points (-2.5,0) and (-15,0)

And there is also a formula to find the mínimum. The mínimum is always the vertex:

Xv = -b/(2*a) = -35/(2*2) = -8.75

Yv = f(Xv) = f(-8.75) = 2(-8.75)^2 + 35(-8.75)  + 75 = -78.125

So the minimum is the point (-8.75, -78.125)

You can easily verify all these data with a graphic software such as GeoGebra.

Here is the complete list of all the points we need to see:

(0,75); (-2.5,0) ; (-15,0), (-8.75, -78.125)

So the mínimum X has to be -15 and the máximum X has to be 0

and the mínimum Y has to be -78.125 and the máximum Y has to be 75

So this leads to a different scale for X and for Y.

Window x = [ -15, 0] y = [ -78.125, 75]