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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)

Sum
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Solution

By the definition of integral,

f(x) = `int f'(x).dx`

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

= `int x  dx - 3 int x^-3 .dx`

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

= `x^2/(2) + (3)/(2x^2) + c`                     ...(1)

f(1) = `(11)/(2)`                                  ...(Given)

∴ `(1)/(2) + (3)/(2) + c = (11)/(2)` 

∴ c = `(7)/(2)`

∴ f(x) = `x^2/(2) + (3)/(2x^2) + (7)/(2)`                   ...[By (1)]

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Chapter 3: Indefinite Integration - Exercise 3.1 [Page 102]

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