3. Determine, with proof, which of the following functions f: R + R are coercive: (a) f(x, y, z) = x+4y-2z for r, y, z E R. (b) f(x, y, z) = a + y + 23- ry for r, y, z e R. (c) f(x, y, 2) = 2 + y? + 22 sin(xyz) for r, y, z E R.