Thursday, September 14, 2017

Limits with Trig



Is there any way to evaluate
limx0xcosxsinxx2


without using L'Hopital's Rule? I was trying to use some of the standard trig limits (e.g. limx0sinxx=1), etc. but couldn't figure it out.




Thank you.


Answer



Adding and subtracting x in nummerator we can split the expression under limit as cosx1x+xsinxx2

The first expression tends to 0 because (1cosx)/x21/2 and the second expression also tends to 0 as shown below.



Since the second expression is an odd function it is sufficient to prove that the expression tends to 0 as x0+. Next note the famous inequality $$\sin x

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