132. Cho hàm số \(y=f\left(x\right)=kx\)( k là hằng số, \(k\ne0\)). Chứng minh rằng:
a) \(f\left(10x\right)=10f\left(x\right)\)
b) \(f\left(x_1+x_2\right)=f\left(x_1\right)+f\left(x_2\right)\)
c) \(f\left(x_1-x_2\right)=f\left(x_1\right)-f\left(x_2\right)\)