先化简,再求值[(a^3+2a^2+4a)/(a^3-8)-10/(a^2+a-6)]/[1-21/(a^2-4)]*(a+3)/(a-2),其中a=1/2

原式=[a(a^2+2a+4)/(a-2)(a^2+2a+4)-10/(a+3)(a-2)]/[(a^2-25)/(a^2-4)*(a+3)/(a-2)]=[a/(a-2)-10/(a+3)(a-2)]/[(a+5)(a-5)(a+3)/(a+2)]=[(a^2+3a-10)/(a-2)]/[(a+2)/(a+5)(a-5)(a+3)]=[(a+5)(a-2)/(a-2)]/[(a+2)/(a...