Let a, b, ybe the angles made by a vector u e r^3 with positive r, y, z-axis respectively. then, the numbers cos a, m = cos cos are called the direction cosines of u. (a) find the formulas for the direction cosines: 1,m, n of the vector u (u1, u2, u3) (b) prove that 12 m2 n2 = 1. (c) find the direction cosines l, m, n of the vector u (1-,1)