For what value of b will the line y=-2x+b be tangent to the parabola y=3x^2+4x-1 ? (Show your work - 2 marks)

Answer 1

I love it when you guys as us to show our work. Ever think of showing some of yours?

y' = 6x+4
The line given has slope -2.
So, when do we have

6x+4 = -2?
x = -1

y(-1) = -2, so we want the point (1,-2) to be on the line. That means

-2 = -2(-1)+b
b = -4

See the graphs at

http://www.wolframalpha.com/input/?i=plot+y%3D3x^2%2B4x-1%2C+y%3D-2x-4