12.已知幂函数f(x)=(m2−m−1)xm2+m−1f(x)=({{m^2}-m-1}){x^{{m^2}+m-1}}f(x)=(m2−m−1)xm2+m−1在(0,+∞)(0,+\infty )(0,+∞)上是减函数,则mmm的值是((( )))