For the first one try to construct a table where you study the sign of each factor -- you're probably looking at this from 0 to 2pi, so make each row to represent the sign of the factors. Your bottom row should be the sign of the overall second derivative. you could also find the intervals or verify by graphing.

For the second one:

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Note that as x goes to zero can be replaced by 5x goes to zero, and

Hope this helps!