# please help with a substitution problem

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• May 2nd 2010, 01:21 PM
rhcp1231
please help with a substitution problem
Evaluate the indefinite integral. (Enter your answer in terms of cos function.)

http://www.webassign.net/cgi-bin/sym...pi%20t%29%20dt

so far i have:

u = 4 $pi$t
du = $pi$dt

1/ $pi$du = dt

once you subsitute it all back in i get a final answer of:

(1/ $pi$)(cos4 $pi$t)+c

but this is not correct....
where is my mistake?
• May 2nd 2010, 01:24 PM
Riyzar
Quote:

Originally Posted by rhcp1231
Evaluate the indefinite integral. (Enter your answer in terms of cos function.)

http://www.webassign.net/cgi-bin/sym...pi%20t%29%20dt

so far i have:

u = 4 $pi$t
du = $pi$dt -> False

1/ $pi$du = dt

once you subsitute it all back in i get a final answer of:

(1/ $pi$)(cos4 $pi$t)+c

but this is not correct....
where is my mistake?

$(4\pi*t)' = 4\pi$

Also note that $\int sin(u)du=-cos(u)+C$
• May 2nd 2010, 01:33 PM
rhcp1231
Quote:

Originally Posted by Riyzar
$(4\pi*t)' = 4\pi$

Also note that $\int sin(u)du=-cos(u)+C$

so is the answer:

-1/pi(cos(pi*t))+c
correct?
• May 2nd 2010, 01:42 PM
skeeter
Quote:

Originally Posted by rhcp1231
so is the answer:

-1/pi(cos(pi*t))+c
correct?

... you're forgetting the 4.

$-\frac{\cos(4\pi t)}{4\pi} + C$