Remember: If a triangle is inscribed in a circle such that the sum of the squares of sides of the triangle is equal to twice the square of diameter, then the triangle is a right-angled triangle whose hypotenuse is the same as the diameter of the circle.
In right ΔABC, ∴sin2A+sin2B+sin2C =sin290∘+sin2B+sin2(90∘−B) =1+sin2B+cos2B =2(sin2θ+cos2θ=1)