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Evaluate the following limit : ablimx→0[ax-bxsin(4x)-sin(2x)]

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Question

Evaluate the following limit : 

`lim_(x ->0) [("a"^x - "b"^x)/(sin(4x) - sin(2x))]`

Sum
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Solution

`lim_(x ->0) [("a"^x - "b"^x)/(sin(4x) - sin(2x))]`

=`lim_(x -  0)[(("a"^x - 1) - ("b"^x - 1))/(sin4x - sin2x)]`

= `lim_(x -> 0) (("a"^x - 1)/x - ("b"^x - 1)/x)/(4* (sin4x)/(4x) - 2* (sin2x)/(2x))`   ...[∵ x→0, ∴ x ≠ 0]

= `(lim_(x -> 0) ("a"^x - 1)/x - lim_(x -> 0) ("b"^x - 1)/x)/(4 lim_(x -> 0) (sin4x)/(4x) - 2 lim_(x -> 0) (sin2x)/(2x))`

= `(log"a" - log"b")/(4(1) - 2(1))  ...[because x -> 0, 4x -> 0, 2x -> 0  "and" lim_(theta -> 0) sintheta/theta = 1]`

= `1/2 log ("a"/"b")`

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Chapter 7: Limits - Exercise 7.6 [Page 154]

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