（ax+by）2+（bx-ay）2,
=a2x2+b2y2+2abxy+b2x2+a2y2-2abxy,
=a2x2+b2x2+b2y2+a2y2,
=（a2+b2）x2+（a2+b2）y2,
=（a2+b2）（x2+y2）．
故答案为：（a2+b2）（x2+y2）．