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Evaluate the following : ∫sinxsin3x.dx - Mathematics and Statistics

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Question

Evaluate the following : `int sinx/(sin 3x).dx`

Sum
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Solution

Let I = `int sinx/(sin 3x).dx`

= `int sinx/(3sinx - 4sin^3x).dx`

= `int (sinx)/(sinx(3 - 4sin^2x)).dx`

= `int (1)/(3 - 4sin^2x).dx`

Dividing both numerator and denominator by cos2x, we get

I = `int (sec^2x)/(3sec^2x - 4tan^2x).dx`

= `int (sec^2x)/(3(1 + tan^2x) - 4tan^2x).dx`

= `int (sec^2x)/(3 - tan^2x).dx`

Put tan x = t

∴ sec2x dx = dt

I = `int dt/(3-t^2)`

I = `int dt/((sqrt(3))^2 - t^2)`

= `int1/((sqrt3)^2 - t^2)dt`

= `(1)/(2sqrt(3)) log |(sqrt(3) + t)/(sqrt(3) - t)| + c`

= `(1)/(2sqrt(3)) log |(sqrt(3) + tanx)/(sqrt(3) - tanx)| + c`.

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Chapter 3: Indefinite Integration - Exercise 3.2 (B) [Page 123]

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