1. Rewrite using the sum-to-product rule.
2. Rewrite as and further using the formula for double angle.
3. Factor out in the left-hand side.
4. To the remaining sum of cosines in the left-hand side, apply the sum-to-product rule.
1. Rewrite using the sum-to-product rule.
2. Rewrite as and further using the formula for double angle.
3. Factor out in the left-hand side.
4. To the remaining sum of cosines in the left-hand side, apply the sum-to-product rule.