a8=4(前羡胡8-1)+a1=27.2a8=( ⊙ o ⊙ )a4+a12,a12=-12.a1=a7-(-1/慧拦3)(派余7-1)=10
解:仔空辩亏悄(1)念缺a8=a1+7d=-1+7*4=27(2)2a8=a4+a12a12=2a8-a4=2*(-4)-4=-12(3)a1=a7-6d=8-6*(-1/3)=8+2=10