If cos (π / 6 - α) is known, then cos (5 π / 6 + α) × sin (2 π / 3 - α) =

If cos (π / 6 - α) is known, then cos (5 π / 6 + α) × sin (2 π / 3 - α) =

The original formula = cos (5 π / 6 + α) * sin (2 π / 3 - α)
=cos[(α+π/3)+π/2]*sin[π-(α+π/3)]
=-sin(α+π/3)*sin(α+π/3)
=-sin²(α+π/3)
=-cos²(α-π/6)