21.已知不等式x2−ax+1<0x^{2}-ax+1<0x2−ax+1<0的解集为{x∣x1<x<x2}\{x\vert x_{1}<x<x_{2}\}{x∣x1<x<x2},且(x1−1)2+(x2−1)2=3(x_{1}-1)^{2}+(x_{2}-1)^{2}=3(x1−1)2+(x2−1)2=3,则a=a=a=[ 3 ].