# tangent line prob

• Jan 7th 2007, 11:35 AM
abcocoa
tangent line prob
:confused: At what point on the graph of y= 1/2x^2 is the tangent line parallel to the line 2x-4x=3?

I took the derivative of the first equation, and set it equal to the slope of the second and got that x was equal to 1/2. Then I plugged that back into the first equation to get that the y coordinate would be 1/8. However, the answer states that the y coordinate is -1/2. the only way to get that is to plug x=1/2 into the second equation. why do you use the second equation to find the point on the graph of the first? Thanks!
• Jan 7th 2007, 12:05 PM
earboth
Quote:

Originally Posted by abcocoa
:confused: At what point on the graph of y= 1/2x^2 is the tangent line parallel to the line 2x-4x=3?

I took the derivative of the first equation, and set it equal to the slope of the second and got that x was equal to 1/2. Then I plugged that back into the first equation to get that the y coordinate would be 1/8. However, the answer states that the y coordinate is -1/2. the only way to get that is to plug x=1/2 into the second equation. why do you use the second equation to find the point on the graph of the first? Thanks!

Hello,

first: There must be a typo in the equation of the line. I assume that you mean: 2x - 4y = 3.
Your results are correct so far.

Maybe you misread the problem? Howexactly is the text of the problem?

EB
• Jan 7th 2007, 03:15 PM
abcocoa
nevermind
the answer in the back of the book was incorrect, so now i know how to do the prob. Thanks anyway.