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【简答题】
已知方程组 x 2 -y+a+2=0 x-y+1=0 的两个 x= x 1 y=y 1 和 x= x 2 y= y 2 ，且x 1 ，x 2 是两个不相等的实数，若x 1 2 +x 2 2 -3x 1 x 2 =8a 2 -6a-11． （1）求a的值； （2）不解方程组判断方程组的两个解能否都是正数？为什么？
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