GATE2016-24

The Laplace transform $F(s)$ of the function $f(t) = \cos (at)$, where $a$ is constant, _________
1. $\dfrac{s^2}{s^2+a^2} \\$
2. $\dfrac{a}{s^2+a^2} \\$
3. $\dfrac{s}{s^2+a^2} \\$
4. $\dfrac{s}{s^2-a^2}$