Sunday, 10 November 2013

calculus - Cannot understand an Integral



π/3π/6dxsecx+cscx=ab2+c2log(d+efg)
I had to solve the integral and get it in this form. However, I cannot understand what to do.
My attempt:

π/3π/6dxsecx+cscx
=π3π6sinxcosxsinx+cosxdx
Substituting t=tan(x2),
tan(π6)tan(π12)2t1+t2×1t21+t2×21+t2dt
213232t(1t2)(1+t2)3dt
Substituting u=1+t2, 2tdt=du, 1t2=2u
243843(2u)u3du
443843duu3243843duu2
After integrating this last step, I had thought I would substitute back u=1+t2=1+tan2(x2). However, this doesn't seem to give me the required value. Could somebody please tell me where I have gone wrong? Also could someone please tell me how to change the limits of the definite integral throughout? This is the first time I have used a Weierstrass Substitution and so I don't know how to change the limits.
Please do help me!

Really, really grateful for your help!


Answer



Here is an alternate method you could use:



Multiply sinxcosxsinx+cosxdx on the top and bottom by cosxsinx to get



cos2xsinx2cos2x1dxsin2xcosx12sin2xdx.



Now substitute u=cosx in the first integral and u=sinx in the second integral to get




u212u2du=12(1+112u2)du=12[u+24ln|1+2u12u|]+C
using partial fractions.



Now you can let u=cosx in the first term and u=sinx in the second to get



12[u+24ln|1+2u12u|]123212[u+24ln|1+2u12u|]3212






Notice that substituting t=tanx2 gives




sinxcosxsinx+cosxdx=2t1+t21t21+t22t1+t2+1t21+t221+t2dt=4t(1t2)(1+t2)2(1+2tt2)dt,



and now you can use partial fractions to continue.


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