How can I solve this question, especially how to rewrite the given expression.

Rewrite the expression as the sine, cosine, or tangent of a double-angle. Then find the exact value of the trigonometric expression without the use of a calculator. 2 cos ( - 22.50) -1 Use the appropriate double-angle formula to rewrite the given expression as the sine, cosine, or tangent of the corresponding double-angle. 2 cos ( - 22.50) -1 = (Do not evaluate. Type your answer in degrees. Do not include the degree symbol in your answer.) Find the exact value of the trigonometric expression without the use of a calculator. 2 cos2( - 22.50) -1 = (Use integers or fractions for any numbers in the expression. Type an exact answer, using radicals as needed.)