# Calc Midterm, First Year University (derivative related stuff)

1. Nov 14, 2008

### y2klimen

1. The problem statement, all variables and given/known data
If f(x)=x^x for x>0, find the constant a such that f'(a)=2f(a)

2. Relevant equations

3. The attempt at a solution
f'(a)=a*a^(a-1)
=a^a

2f(a)=2a^a

2a^a=a^a
ln2a^a=lna^a
aln2a=alna

....?????

2. Nov 14, 2008

### snipez90

If f(x) = x^x, then f'(x) =/= x*x^(x-1). That rule works if your exponent is a number, not a function. Rewrite x^x in exponential form and then differentiate using the chain rule to find f'(x).

3. Nov 15, 2008

### HallsofIvy

Staff Emeritus
No that is not right. You cannot treat a (x) as a variable in one case (the base) and a constant in the other (the exponent).

If y= xx then ln(y)= x ln(x). Use the chain rule to find dy/dx.