When to use Q=UI sin(a) vs Q=I²R sin²(a) for reactive power?

[Gavroy]

hey,
i am a little bit confused about the reactive power.
we defined it as
Q=UI sin(a)
and the apparent power as
S=UI

Now I have to solve this exercise:
Which percentage of heat loss in a power grid is caused by the reactive power, when the powerfactor is cos(a)=0,85?

Now I thought:
Q/S=sin(a) would be the solution

but instead of this it is said that one has to set:
Q= I² R sin²(a)
S= I² R

and therefore Q/S=sin²(a)

now my question is: when can I use Q=U I sin(a) and when do I have to use Q= I² R sin²(a)...can somebody explain this to me?

 

[Naty1]

Put it in the homework section