Thread: For what values of a and b is the line 4x + y = b tangent to the parabola y = ax2 wh

__a=1/2
__b=-8
__a=-1/2
__b=8
__a=-20

Originally Posted by plstevens

__a=1/2
__b=-8
__a=-1/2
__b=8
__a=-20

what does wh in your question mean?

are the list of a's and b's you gave options to choose?

the wh, should be when x=4, and the a's and b's are th choices that i was given

Originally Posted by plstevens

the wh, should be when x=4, and the a's and b's are th choices that i was given

Well, given the parabola , what is the equation of the line tangent to it at x = 4?

The tangent line must pass through the point (4, 16a).

, so when x = 4 the slope of the tangent line is .

So the equation of the line that passes through the point (4, 16a) that has a slope of 8a is:





Insert the point (4, 16a) to find b:




So the tangent line is or in the form given in the problem:


Compare this with


What are your values for a and b?

-Dan