Tutor help me.. how to solve this problem to understand this.. Thank you so much

Problem linear Algebra Read Chapter v $1 in the text. Let m n be non-negative integers. Show that cos(mz) cos(nx) dr = 0. Let n = 1, 2,3, .... Show that cos ? ( na ) dr = n. For any fixed n, show that the functions cos(x), cos(2x), cos(3x), ... cos(nx) are linearly independent