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If f'(x) = x-3x3, f(1) = 112 find f(x)

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Question

If f'(x) = `x - 3/x^3`, f(1) = `11/2` find f(x)

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Solution

f'(x) = `x - 3/x^3`, f(1) = `11/2`     .........[Given]

f(x) = `int"f'"(x)  "d"x`

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

= `int x  "d"x - 3 int x^(-3) "d"x`

= `x^2/2 - 3 (x^(-2)/2) + "c"`

∴f(x) = `x^2/2 + 3/(2x^2) + "c"`

∴ f(1) = `(1)^2/2 + 3/(2(1)^2 + "c"`

∴ `11/2 = 1/2 + 3/2 + "c"`

∴ `11/2` =2 + c

∴ c = `7/2`

Substituting c = `7/2` in (i),, w get

f(x) = `x^2/2 + 3/(2x^2) + 7/2`

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Chapter 2.3: Indefinite Integration - Short Answers II

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