f(x)=∫(a,x)(t-1)dtf'(x)=x-1=0x=1f''(x)=1>0所以x=1时取极小值=f(1)=∫(a,1)(t-1)dt=(t-1)²/2|(a,1)=0-(a-1)²/2=-(a-1)²/2
