Sunday, 3 March 2013

calculus - How to evaluate the limlimitsxto0frac2sin(x)arctan(x)xcos(x2)x5, using power series?

How to evaluate the limx02sin(x)arctan(x)xcos(x2)x5, using power series?



It made sense to first try and build the numerator using power series that are commonly used:



2sin(x)=k=02(1)kx2k+12k+1!=2xx33+x660+




arctan(x)=k=0(1)k+1x2k+12k+1=x+x33x66+



xcos(x2)=k=0(1)k+1x4k+12k!=x+x52+



Hence,



limx02sin(x)arctan(x)xcos(x2)x5=limx0[2xx33+x660+]+[x+x33x66+]+[x+x52+]x5




In similar problems, the there is an easy way to take out a common factor that would cancel out with the denominator, resulting in an easy-to-calculate limit. Here, however, if we were to take a common factor from the numerator, say, x6, then we would end up with an extra x



What possible strategies are there to solve this question?

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