2. Because A+C=2B? So b = 60? According to cosine theorem b 2? =? a^2？ +? c^2？ -? 2 a c cosB？ Found that c equals 2.

So the triangle is a right triangle? C=90？ So the value of sinC is 1.

3. as shown in the figure? You know 2R=a\sinA? Similarly, other angles can also prove that Sina is a=sinB, b=sinC and c.