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Evaluate the following limit : limx→0[cos(ax)-cos(bx)cos(cx)-1] - Mathematics and Statistics

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प्रश्न

Evaluate the following limit :

`lim_(x -> 0) [(cos("a"x) - cos("b"x))/(cos("c"x) - 1)]`

बेरीज
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उत्तर

`lim_(x -> 0) (cos("a"x) - cos("b"x))/(cos("c"x) - 1)`

= `lim_(x -> 0) (-cos("a"x) + cos("b"x))/(1 - cos("c"x))`

= `lim_(x -> 0) (1 - cos "a"x - 1 + cos "b"x)/(1 - cos "c"x)`

= `lim_(x -> 0) ((1 - cos"a"x) - (1 - cos"b"x))/(1 - cos"c"x)`

= `lim_(x -> 0) (2sin^2  (("a"x)/2) - 2sin^2  ("b"x)/2)/(2sin^2  ("c"x)/2`

= `lim_(x -> 0) ((sin^2 (("a"x)/2) - sin^2  (("b"x)/2))/(x^2))/((sin^2 (("c"x)/2))/(x^2))   ...[("Divide numerator and denominator by"  x^2),(because x -> 0"," therefore x ≠ 0 therefore x^2 ≠ 0)]`

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

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

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

= `((1)^2 * "a"^2/4 - (1)^2 * "b"^2/4)/((1)^2 * "c"^2/4`

= `("a"^2/4 - "b"^2/4)/("c"^2/4)`

= `("a"^2 - "b"^2)/"c"^2`

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पाठ 7: Limits - Exercise 7.4 [पृष्ठ १४८]

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