Recall that [tex]\cos x[/tex] is bounded between -1 and 1 for all real [tex]x[/tex], so that
[tex]-1\le \cos(5t)\le 1[/tex]
[tex]\implies-\dfrac1{5t}\le\dfrac{\cos(5t)}{5t}\le\dfrac1{5t}[/tex]
Now take the limit of each side of the inequality:
[tex]\displaystyle\lim_{t\to\infty}\left(-\dfrac1{5t}\right)\le\lim_{t\to\infty}\dfrac{\cos(5t)}{5t}\le\lim_{t\to\infty}\dfrac1{5t}[/tex]
The bounding limits are both 0, so by the squeeze theorem the desired limit is also 0.
