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Evaluate: d∫09xx+9-x dx

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Question

Evaluate: `int_0^9 sqrt(x)/(sqrt(x) + sqrt(9 - x)  "d"x`

Sum
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Solution

Let I = `int_0^9 sqrt(x)/(sqrt(x) + sqrt(9) - x)  "d"x`   ........(i)

= `int_0^9 (sqrt(9 - x))/(sqrt(9 - x) + sqrt(9 - (9 -x)))  "d"x`     ........`[∵ int_0^"a" "f"(x)"d"x = int_0^"a" "f"("a" - x)"d"x]`

∴ I = `int_0^9 (sqrt(9 - x))/(sqrt(9 - x) + sqrt(x))  "d"x`  ......(ii)

Adding (i) and (ii), we get

2I = `int_0^9 (sqrt(x))/(sqrt(x) + sqrt(9 - x))  "d"x + int_0^9  (sqrt(9 - x))/(sqrt(9 - x) + sqrt(x))  "d"x`

= `int_0^9 (sqrt(x) + sqrt(9 - x))/(sqrt(x) + sqrt(9 - x))  "d"x`

= `int_0^9 "d"x`

= `[x]_0^9`

∴ 2I = 9 − 0

∴ I = `9/2`

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Methods of Evaluation and Properties of Definite Integral
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Chapter 2.4: Definite Integration - Short Answers I

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