For the formula 1, the right side brings in the degree of a, and after the left side divides it equally, you get

After the value of formula 2 is brought in, sinBsinC= 1 is obtained.

Solve this formula and formula 2 at the same time, and you can get sinB=sinC= 1/2.

In fact, there are many such problems on the Internet, but only a little change in numerical value is different. I suggest you search for a few questions to consolidate, go in and ask the answer home page, and you can probably find many similar questions by typing.