【高三数学】不等式基本性质的证明题》》》

(px+qy)²-(px²+qy²)=(p²-p)x²+(q²-q)y²+2pqxy
=p(p-1)x²+q(q-1)y²+2p(1-p)xy
=p(p-1)x²+(1-p)(-p)y²-2p(p-1)xy
=p(p-1)x²+(p-1)py²-2p(p-1)xy
=p(p-1)(x-y)²=-pq(x-y)²