lim(x->0){[f(3t)-f(-t)]/(2t)}
=lim(x->0){[(3/2)(f(3t)-f(0))/(3t)]+[(1/2)(f(-t)-f(0))/(-t)]}
=(3/2)lim(x->0)[f(3t)-f(0))/(3t)]+(1/2)lim(x->0)[f(-t)-f(0))/(-t)]
=(3/2)f'(0)+(1/2)f'(0)
=(3/2)*1+(1/2)*1
=2.
lim(x->0){[f(3t)-f(-t)]/(2t)}
=lim(x->0){[(3/2)(f(3t)-f(0))/(3t)]+[(1/2)(f(-t)-f(0))/(-t)]}
=(3/2)lim(x->0)[f(3t)-f(0))/(3t)]+(1/2)lim(x->0)[f(-t)-f(0))/(-t)]
=(3/2)f'(0)+(1/2)f'(0)
=(3/2)*1+(1/2)*1
=2.