# Help with Integral proof

• Jan 21st 2009, 04:15 PM
jdhami
Help with Integral proof
I'm confused about this question so some help would be appreciated.

"Suppose f is an even function and g is an odd function. Explain for any a>0, Integral (from -a to a) f =2 Integral (from 0 to a) f and Integral (from -a to a) g=0.
First make a sketch of such functions and look at net areas. Can you also explain these results with the Fundamental Theorem of Calculus?
(hint: if a function is even (or odd) then its derivative is odd (or even); and if a function is
odd then any anti-derivative is even and if a function is even then the anti-derivative that passes through the origin is odd (but not the other anti-derivatives!)).

I have no clue how to proceed with this question. I tried graphing even and odd functions. I graphed y=x^2 as well as y=x^4 and I noticed that the graphs had the same magnitude of area from the origin to a and -a, respectively. Odd functions, I used y=x^3 and x^5 and these graphs had the same area from -a to 0 and 0 to a but opposite sides of the x-axis. However, I can't seem to piece together what this means so any help would really help me out.
• Jan 21st 2009, 06:04 PM
vincisonfire
The hint is a really good one.
Even function $\displaystyle f(a)=f(-a)$
Odd function $\displaystyle -f(a)=f(-a)$
Fundamental theorem of calculus $\displaystyle \int_{a}^b f(x) = F(b)-F(a)$
When you integrate an even function then you obtain an odd function.
$\displaystyle \int_{-a}^a f_{even}(x) = F_{odd}(a)-F_{odd}(-a)=F_{odd}(a)+F_{odd}(a)=2F_{odd}(a)$
When you integrate an odd function then you obtain an even function.
$\displaystyle \int_{-a}^a f_{odd}(x) = F_{even}(a)-F_{even}(-a)=F_{even}(a)-F_{even}(a)=0$
• Jan 21st 2009, 06:09 PM
jdhami
Thank you! I managed to get it to work graphically and that helped me confirm my proof via the FTC!