根据题意得x1+x2= 7 4 ，x1x2=- 3 4 ，（1）原式=x1x2-3（x1+x2）+9=- 3 4 -3× 7 4 +9=3；（2）原式= x12+x22 x1x2 = (x1+x2)2?2x1x2 x1x2 = ( 7 4 )2?2×(? 3 4 ) ? 3 4 =- 73 12 ．
