a3+a10=a5+a8=18a2+a11=a5+a8=18所以原式=18+18=36
a2+a3+a10+a11=(a5-3d)+(a5-2d)+(a8+2d)+(a8+3d)=2(a5+a8)=2×18=36
a1+a12=a3+a10=a5+a8则原式=36
