HELP!! I'm not sure what goes in the blank.

10β

Step-by-step explanation:

We can find this two ways, first by seeing in the step after it, cosines are canceled out. Since you already have 10β on the next step, you can assume that (since only the cosines changed and the cosine next ot the blank was removed), the value is 10β.

You can also use double angle formulas from the previous step:

(sin(2β) = 2 sin(β) cos(β))and find that:

5 sin (2β) sin(β) = 5 * (2 sin(β) cos(β)) sin(β)) = (10 sin(β) sin(β)) cos(β) =

10β cos(β)

But since cos(β) is already present, we can see that the answer is 10β