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What is the definite integral of (sec^4 x) from 0 to pi/4?  Just for clarification,...

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bandgeek2339 | Student, College Freshman | eNotes Newbie

Posted September 16, 2009 at 11:25 PM via web

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What is the definite integral of (sec^4 x) from 0 to pi/4?  Just for clarification, that is a secant to the fourth power of x.

I had this question on a quiz, and I "saved" a sec^2x and then made the other sec^2x equal to (tan^2x + 1) and then let u = tanx. I was able to solve that indefinitely, but the problem comes about because tan(0) is undefined and that is the lower limit of integration.  Any help would be much appreciated--This problem is going to bothering me all day until I get my quiz back! :)

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neela | High School Teacher | (Level 3) Valedictorian

Posted September 16, 2009 at 11:48 PM (Answer #1)

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(secx)^4  = (1+tan^2 x) sec^2

Therefore Integral (sec^4 x) dx = Integral (1+tan^2 x) sec^2 x dx

Let tan x=t , then sec^2 x dx = dt , for x=0, t=0  and for x=pi/4 , t=1, with this transformation,

Right side is  integral (1+t^2) dt

=(t+t^3/3)+C . Taking  itegral limits from t= 0 to t=1 , we get

=(1+1^3/3+C)- (0+0+C)

=4/3

NB : Tan 0 = 0 and it is not undefined, whereas tan 90 = is undefined with change of sign and infinite jump from  +inf to -inf as go from 90-  to  90+.

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