GKFix MCQ Question

Which trigonometric identity is used to derive the beats equation from s = a(cos ω₁t + cos ω₂t)?

• A. sin A + sin B = 2 sin((A+B)/2) cos((A–B)/2)
• B. cos A + cos B = 2 sin((A+B)/2) sin((A–B)/2)
• C. cos A + cos B = cos(A+B) + cos(A–B)
• D. cos A + cos B = 2 cos((A+B)/2) cos((A–B)/2)