How to evaluate the limx→02sin(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!=2x−x33+x660+⋯
−arctan(x)=∞∑k=0(−1)k+1x2k+12k+1=−x+x33−x66+⋯
−xcos(x2)=∞∑k=0(−1)k+1x4k+12k!=−x+x52+⋯
Hence,
limx→02sin(x)−arctan(x)−xcos(x2)x5=limx→0[2x−x33+x660+⋯]+[−x+x33−x66+⋯]+[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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