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∫xcos3x dx - Mathematics and Statistics

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प्रश्न

`int xcos^3x  "d"x`

योग
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उत्तर

Let I = `int xcos^3x  "d"x`

cos3x = 4cos3x − 3cosx

∴ 4cos3x = 3cos x + cos 3x

∴ cos3x = `1/4 (3cos x + cos 3x)`

∴ I = `1/4 int x (3cos x + cos 3x)  "d"x`

= `1/4[x int (3cosx + cos3x) "d"x - int{"d"/("d"x)(x) int(3cos x + cos 3x)"d"x}"d"x]`

= `1/4[x(3sinx + (sin3x)/3) - int 1(3sinx + (sin3x)/3)"d"x]`

= `1/4[3x sinx + x/3 sin 3x - (-3 cosx - 1/3 * (cos3x)/3)] + "c"`

∴ I = `1/4(3x sinx + x/3 sin 3x + 3 cos x + 1/9 cos 3x) + "c"`

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अध्याय 2.3: Indefinite Integration - Long Answers III

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