(sinA-sinB)^2 + (cosA-cosB)^2
= sinA^2 - 2sinAsinB + sinB^2 + cosA^2 - 2cosAcosB + cosB^2
= sinA^2 + cosA^2 + sinB^2 + cosB^2 - 2(cosAcosB+sinAsinB)
= 2 - 2cos(A-B)
= 1/4 + 1/4 = 1/2
∴ cos(A-B) = 3/4