http://img29.imageshack.us/img29/1954/capturend.png

Maybe we can walk through this? At first glance, the first thing I would do is find the derivative of , but that's not right.

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- February 4th 2010, 04:45 PMBeSweeetFinding k of a Tangent Line
http://img29.imageshack.us/img29/1954/capturend.png

Maybe we can walk through this? At first glance, the first thing I would do is find the derivative of , but that's not right. - February 4th 2010, 04:50 PMpickslides
- February 4th 2010, 04:57 PMBeSweeet
- February 4th 2010, 05:03 PMpickslides
- February 4th 2010, 05:08 PMBeSweeet
- February 5th 2010, 03:46 AMHallsofIvy
If you haven't learned "the whole dy/dx thing", why did you refer to finding the derivative in your first post? The formula you give is for the slope of a straight line. If f(x) is not linear, it is the slope of a "secant" line, a line that crosses the graph at x and x+y, not the tangent line. Since you don't yet know the derivative (I suspect this problem is preliminary to introducing the derivative), here's a method that predates the calculus:

If y= 5x+ 45 and meet at all, we must have or . Let u= so that . The equation becomes . If x= a at the point of intersection then x- a= must be a factor of that polynomia. If fact, to be**tangent**that must be a**double**factor- we must have Multiplying the right side, we get for all u. Comparing coefficients, and . Solve the second equation for a and use that to solve the first equation for k.