suppose u(x1,x2) and v(x1,x2) are utility functions.
(1)Prove that if u(x1,x2) and v(x1,x2) are homogeneous of degree r,
    then s=u(x1,x2) + v(x1,x2) is  homogeneous of degree r.
(2)Prove that if u(x1,x2) and v(x1,x2) are quasiconcave,
    then min(x1,x2)=min{u(x1,x2) , v(x1,x2)} is  also quasiconcave.