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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

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  • 1 decade ago
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    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

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