3. If K(x, y) and L(x,y) are valid kerel functions, show that: (a) M(x, y) K(x, y) + L(x, y) is also a kernel (b) N(x,y) ak(x,y), a > 0 is also a kernel (c) P(x,y) - K(x, y)L(x, y) is also a kernel