Trigonometry
posted by Tony .
In a triangle ABC, if 2sinB=sinA+sinC and sides a+c=70, what is side b?

b/sinB = a/sinA = c/sinC
sinA/a = sinC/C
sinA = a/c sinC
sinA+sinC = (1+a/c) sinC
= (a+c)/c sinC
2sinB = sinA+sinC = 70/c sinC
2b/2sinB = c/sinC
2b = (70/c) sinC * c/sinC
= 70
b = 35