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∫sinxsin3x dx

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प्रश्न

`int (sinx)/(sin3x)  "d"x`

योग
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उत्तर

Let I = `int (sinx)/(sin3x)  "d"x`

= `int sin x/(3sin x - 4 sin^3x)* "d"x`

= `int sinx/(sinx(3 - 4sin^2x))* "d"x`

= `int 1/(3 - 4sin^2x)  "d"x`

Dividing numerator and denominator by cos2x, we get

I = `int (sec^2x)/(3sec^2x - 4tan^2x) * "d"x`

= `int (sec^2x)/(3(1 + tan^2x) - 4tan^2x)* "d"x`

= `int (sec^2x)/(3 - tan^2x)  "d"x`

Put tan x = t

∴ sec2x dx = dt

∴ I = `int "dt"/(3 - "t"^2)`

= `int 1/((sqrt(3))^2 - "t"^2)  "dt"`

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

∴ I = `1/(2sqrt(3)) log|(sqrt(3) + tanx)/(sqrt(3) - tanx)| + "c'` 

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अध्याय 2.3: Indefinite Integration - Short Answers II

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