Find the maximum of [MATH]f_n[/MATH] on [MATH][0,\infty)[/MATH], by differentiating, setting equal to 0 and solving for x.
Then show that [MATH](f_n)[/MATH] converges uniformly to [MATH] f=0[/MATH], on [MATH] [0,\infty)[/MATH] by using the definition of uniform convergence and choosing a suitable value for 'N'.