22.先化简,再求值:[(3a-7)²-(a+5)²]/(4a-24),其中a=1/50

题目

22.先化简,再求值:[(3a-7)²-(a+5)²]/(4a-24),其中a=1/50

答案

[(3a-7)^2-(a+5)^2]/(4a-24)=[(9a²-42a+49)-(a²+10a+25)]/(4a-24)=(8a²-52a+24)/[4(a-6)]=(a-6)(8a-4)/[4(a-6)]=(8a-4)/4=2a-1=2*(1/50)-1=1/25-1=-24/25