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Evaluate: ∫-111a2ex+b2e-x - Mathematics and Statistics

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Question

Evaluate: `int_(-1)^1 1/("a"^2"e"^x + "b"^2"e"^(-x))  "d"x`

Sum
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Solution

Let I = `int_(-1)^1 1/("a"^2"e"^x + "b"^2"e"^(-x))  "d"x`

= `int_(-1)^1 1/("a"^2"e"^x + ("b"^2)/("e"^x))  "d"x`

= `int_(-1)^1 "e"^x/("a"^2("e"^x)^2 + "b"^2)  "d"x`

Put ex = t

∴ ex dx = dt

When x = −1, t = e−1 and when x = 1, t = e

∴ I = `int_("e"^-1)^"e"  "dt"/("a"^2"t"^2 + "b"^2)`

= `1/("a"^2) int_("e"^-1)^"e"  "dt"/("t"^2 + ("b"/"a")^2`

= `1/("a"^2)[1/("b"/"a")tan^-1 ("t"/("b"/"a"))]_("e"^-1)^"e"`

= `1/"ab"[tan^-1("at"/"b")]_("e"^-1)^"e"`

∴ I = `1/"ab"[tan^-1("ae"/"b") - tan^-1("a"/"be")]`

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

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