先化简,再求值[(a^3+2a^2+4a)/(a-2)(a^2+2a+4)]-10/(a^2+a-6)除以[1-21/(a^2-4)]*(a+3)/(a-2),其中a=1/2

(a^3+2a^2+4a)/(a-2)(a^2+2a+4)=[a*(a^2+2a+4)]/(a-2)(a^2+2a+4)=a/(a-2)
10/(a^2+a-6)=10/[（a+3）*(a-2)]
[(a^3+2a^2+4a)/(a-2)(a^2+2a+4)]-10/(a^2+a-6)=a/(a-2) -10/(a^2+a-6)=(a+5)/(a+3)
[1-21/(a^2-4)]*(a+3)/(a-2)=[(a^2-25)*(a+3)]/[(a^2-4)*(a-2)]
全等式=（a-2）*(a-4)^2除以（a+5）*(a+3)^2=(0.5-2)/(0.5-5)=1/3