$\displaystyle \lim x \rightarrow \infty[x sin(\dfrac{5\pi}{x})$
$\displaystyle \lim x \rightarrow \infty[\infty sin(\dfrac{5\pi}{\infty})$ Any starting hint here?
Just as an additional comment...In your two posts in this topic you have put x = infinity for your last line. Infinity is the limit...it is not a number.