Part 5/6:
- Expand the products and utilize algebraic manipulation to observe:
- Change in sign in the sine components results in differentiating cosine interactions with the angles concerned.
Memorization Tips
To aid remembering these formulas, one useful technique is to note how the signs change based on addition and subtraction:
For the addition formula: cos x cos y - sin x sin y
For the subtraction formula: cos x cos y + sin x sin y
This switch in sine's sign provides a practical way to memorize the difference between addition and subtraction.