a1=s1=2*1^2+1-1=2sn=2n^2+n-1s(n-1)=2(n-1)^2+(n-1)-1=2n^2-4n+2+n-1-1=2n^2-3nan=sn-s(n-1)=2n^2+n-1-(2n^2-3n)=2n^2+n-1-2n^2+3n=3n-1an是以-1为首项,公差为3的等差数列a5=3*5-1=14a3+a4+a5+a6+a7=5a5=5*14=70...为什么是3n-1不是4n-1？不好意思确实是4n-1an=sn-s(n-1)=2n^2+n-1-(2n^2-3n)=2n^2+n-1-2n^2+3n=4n-1an是以-1为首项，公差为4的等差数列a5=4*5-1=19a3+a4+a5+a6+a7=5a5=5*19=95为什么要算出a5？等差数列,求a5事省a3+a4+a5+a6+a7=a3+a7+a4+a6+a5=2a5+2a5+a5=5a5