math
posted by Jaime .
Hi
This is a threepart question:
a. the graph y = f(x) in the xyplane has parametrization x=x, y=f(x), and vector formula r(x) = xi + f(x)j. Use this to show that if f is twicedifferentiable, then
((absvalue f''(x))/[1+((f'(x))^2]^3/2
b. use the formula for k in part a to find the curvature of y = ln(cosx) when pi/2 < x < pi/2.
c. show that the curvature is zero at the point of inflection.

You left out something in (a) There is no = sign. It looks like you are trying to compute the curvature of the y = f(x) line, which is defined as the reciprocal of the radius of curvature, or d(phi)/ds, where phi is the slope and x is the arc length.
I suggest you review derivations such as the ones at
http://books.google.com/books?id=chjoVpzLZ2UC&pg=PA203&lpg=PA203&dq=curvature+%22dy+dx%22&source=web&ots=mCjWZsZZxM&sig=5T1SzRY3HMvrctecDnY7KMfZJjc
or
http://www.everything2.com/index.pl?node_id=1290875