Integration by trig substitution

  • Thread starter mvantuyl
  • Start date
  • #1
37
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Homework Statement


Integrate: [tex]\int[/tex][tex]\sqrt{1-9t^{2}}[/tex]dt


Homework Equations





The Attempt at a Solution


t = 1/3 sin[tex]\Theta[/tex]
dt/d[tex]\Theta[/tex] = 1/3 cos[tex]\Theta[/tex]
dt = 1/3 cos[tex]\Theta[/tex]d[tex]\Theta[/tex]
3t = sin[tex]\Theta[/tex]

1/3[tex]\int\sqrt{1-sin^{2}}\Theta[/tex] cos[tex]\Theta[/tex]d[tex]\Theta[/tex]
1/3[tex]\int cos^{2}\Theta[/tex]d[tex]\Theta[/tex]
1/3[tex]\int[/tex](1 + cos 2[tex]\Theta[/tex]) / 2 d[tex]\Theta[/tex]
1/6[tex]\int1 + cos2\Theta[/tex] d[tex]\Theta[/tex]
1/6([tex]\Theta + 1/2 sin 2\Theta[/tex]) + C
1/6([tex]\Theta + 1/2(sin\Theta cos\Theta[/tex]) + C
1/6([tex]\Theta + 1/2(sin\Theta\sqrt{1-sin^{2}\Theta}[/tex])) + C
1/6([tex]\Theta[/tex] + 1/2(3t[tex]\sqrt{1-9t^{2}}[/tex])) + C

I can't figure out how to get rid of [tex]\Theta[/tex] in the result.
 

Answers and Replies

  • #2
1,100
0
Why not using arcsin? Also, there is a factor of two missing in the 3rd step from the end.

The Bob
 
  • #3
37
0
Perfect! Thanks.
 

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