# Slope of the perpendicular line question

• February 9th 2011, 04:07 PM
youngb11
Slope of the perpendicular line question
Question:

At what point on the curve $y=-2x^4$ is the tangent line perpendicular to the line $x-y+1=0$?

What I did was set the derivative of the curve ( $-8x^3$) equal to the negative slope of the second (-1 in this case) as that's how I assumed we find the perpendicular line.

I got an answer of 2, but the book says $x=1/2$
• February 9th 2011, 04:10 PM
$-8x^3 = -1$
$8x^3 = 1$
$x^3 = 1/8$
$x = 1/2$