lim(x→0) f(3x)/2=3/2 lim(x→0) f(3x)=3
x/2=3*(x/6) x'=x/6
lim(x→0) f(x/2)/sinx=lim(x'→0) f(3x')/sin6x'
x'→0,sin6x'→0
lim(x→0)f(x/2)/sinx极限不存在不对