Help how to do this limit

Is aX^(-2)b = abX^(-2)? Please clarify.

2011-12-05 11:03:59 補充：
Make use of the sin x power series,
sin 3x = (3x) - (3x)^3/3! + (3x)^5/5! + .......
so (sin 3x)/x^3 = 3/x^2 - 3^3/3! + 3^5 x^2/5! + ........
so sin 3x/x^3 + a/x^2 + b = 3/x^2 - 3^3/3! + 3^5 x^2/5 + a/x^2 + b
For limit to be zero, 3/x^2 + a/x^2 = 0, so a = - 3
and - 3^3/3! + b = 0, so b = 3^3/3! = 9/2
so a + b = - 3 + 9/2 = 3/2

2011-12-05 14:18:12 補充：
WolframAlpha uses L'Hospital Rule to solve it but looks very complicated, may be someone can make it easier.