text: The coefficients of Chebyshev polynomials of the first kind can be found using identities involving this function.
full_question: The coefficients of Chebyshev polynomials of the first kind can be found using identities involving this function. This function of a complex number z is the arithmetic mean of e to the i times z and that exponential's reciprocal. This function is on the main diagonal of a two-dimensional rotation matrix. A complex number's magnitude times this function of its argument yields its (*) real part. This even function is equal to its own fourth derivative, and its Taylor expansion begins 1 minus x-squared over 2. c-squared equals a- squared plus b-squared minus 2 a b times this function of angle C, according to its namesake law in triangles. For 10 points, identify this trigonometric function, which for an angle in a right triangle equals adjacent over hypotenuse.
first_sentence: The coefficients of Chebyshev polynomials of the first kind can be found using identities involving this function.
raw_answer: cosine