# Homework Help: How to proove that that e^x is convex

1. Oct 2, 2011

### Rabolisk

1. The problem statement, all variables and given/known data

I have to determine if [e][/x] is a convex function. If it is then show proof. I know its a convex function by looking at the graph, Iam stuck at prooving it mathematically though.

2. Relevant equations

The function is f(x)=e^x.

3. The attempt at a solution
I am certain that the function is convex. I'am having trouble proving it though.

Assuming we pick a point and call it x0, then a lies to the left of x0 and point b lies to the right.

f(ta+(1-t)b)<=t*f(a)+(1-t)*f(b)

Once we substitute we get.
e^((ta+(1-t)b))<=t*e^a+(1-t)*e^b

I'am stuck at proving how this inequality is true.

Thanks!

2. Oct 2, 2011

### HallsofIvy

If you are required to use the definition of "convex" then use the fact that
$$e^{ta+ (1-t)b}= e^{ta}e^be^{-bt}$$

Or are you allowed to use the fact that a function is convex if and only if its second derivative is positive for all x?

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