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How can I evaluate these without integration tables?
I'll use "Int" for the indefinite integral symbol:
Int[(tanx)^3(secx)^3]dx
Int[(cos(3x))^4]dx
1 Answer
- anakin_louixLv 61 decade agoFavorite Answer
Ok, Let's go with the first one :
Int[(tanx)^3(secx)^3]dx
Remember : tan^2x + 1 = sec^2x
int[tanx*(sec^2x - 1)*sec*3x]dx
int[tanx*(sec^5x - sec^3x)]dx
remember : tanx = sinx / cosx
secx = 1 / cosx
int[(sinx / cos^6x)dx - int[sinx / cox^4x]dx
Now we have easier integrals, remember : - d(cosx) = sinx
-int[dcosx / cos^6x] + int[dcosx / cos^4x]dx
1/5*(cosx)^-5 - 1/3*(cosx)^-3
That's it
Second problem :
Int[(cos(3x))^4]dx
Remember :
2*cos^2(3x) = cos6x + 1
cos^2(3x) = (cos6x + 1) / 2
cos^4(3x) = ( 1 + cos^2(6x) + 2*cos6x ) / 4
Let's replace it on the integral :
int[ ( 1 + cos^2(6x) + 2*cos6x ) / 4)]dx
1/4*int[dx] + 1/4*int[cos^2(6x)] + 1/2*int(cos6x)
1/4*x + 1/12*sin(6x) + 1/4*int[cos^2(6x)]dx.......(**)
This : 1/4*int[cos^2(6x)]dx is simple :
1/8*int[2cos^2(6x)]dx
1/8*int[cos(12x) + 1]dx
1/64*sin(12x) + x/8 ..... (*)
Let's replace : (*) on (**)
1/4*x + 1/12*sin(6x) + 1/64*sin(12x) + x/8
That's it
Hope that helps