Is the following statement true or false: cos A cos B = cos AB? Explain.

Answer 1

You may also prove this using a counterexample. It only takes one example that shows they are not equal for you to say it is not a correct equation.

For instance, say A = 10⁰ and B = 45⁰, then AB = 45⁰. Then substitute;

(cos 10)(cos45) = cos 450.... is this a true statement?

Answer 2

No, refer to the product to sum formulas.