θ→0
lim
​
θ 2
sin(4θ)sin(5θ)
​
​
= θ→0
lim
​
θ
sin(40)
​
⋅ 4
4
​
⋅ θ→0
lim
​
θ
sin(5θ)
​
⋅ 5
5
​
=4⋅5=20
​
please explain this. what is this shortcut? why is there a 4/4 and 5/5 multiplied?