lim(x->0) (cos5x-cos3x)/5x^2=
lim(x->0) (-2sin4x*sinx)/5x^2=
-2/5 lim(x->0) (sin4x/x)*(sinx/x)=
-2/5 lim(x->0) (4sinx/4x)*(sinx/x)=-2/5 *1*1=-2/5
lim(x->0) (cos5x-cos3x)/5x^2=
lim(x->0) (-2sin4x*sinx)/5x^2=
-2/5 lim(x->0) (sin4x/x)*(sinx/x)=
-2/5 lim(x->0) (4sinx/4x)*(sinx/x)=-2/5 *1*1=-2/5