Math

Express as a single trigonometric function: cosine 20 degrees cosine 15 degrees plus sine 20 degrees sine 15 degrees. What is the derivative of cotangent x with respect to x? Without using negative exponents, express the indefinite integral of 10 over x squared with respect to x. The vectors (3, -5) and (x,6) are perpendicular. Find the value of x. ................
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