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Integrals

Question
CBSEENMA12032469

Evaluate the following integral:
integral subscript straight pi over 6 end subscript superscript straight pi over 2 end superscript fraction numerator cosec space straight x space cotx over denominator 1 minus cosec squared straight x end fraction dx


Solution

Let I = integral subscript straight pi over 6 end subscript superscript straight pi over 2 end superscript fraction numerator cosecx space cosx over denominator 1 plus cosec squared straight x end fraction dx
Put cosec x = y,    therefore space space space space minus cosecx space cot space straight x space dx space equals space dy         rightwards double arrow space space cosec space straight x space cotx space dx space equals space minus dy
When straight x equals straight pi over 6 comma space space space space space straight y space equals space cosec straight pi over 6 space equals space 2
When straight x equals straight pi over 2      straight y equals cosec straight pi over 2 space equals space 1
therefore space space space straight I space equals space space integral subscript 2 superscript 1 fraction numerator 1 over denominator 1 plus straight y squared end fraction dy space equals space minus open square brackets tan to the power of negative 1 end exponent space straight y close square brackets subscript 2 superscript 1 space equals space minus left parenthesis tan to the power of negative 1 end exponent 1 space minus space tan to the power of negative 1 end exponent 2 right parenthesis space equals space tan to the power of negative 1 end exponent 2 space minus space tan to the power of negative 1 end exponent 1
           equals space tan to the power of negative 1 end exponent open parentheses fraction numerator 2 minus 1 over denominator 1 plus 2 end fraction close parentheses space equals space tan to the power of negative 1 end exponent 1 third

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