Question

已知等差数列{an}满足：a3=7,a5+a7=26.

Answer 1

=>a1=3,d=2 ==>Sn=n^2+2n

=>Tn=(1*1+2*2+3*3+...+n*n)+(2+4+6+...+2n)

=>Tn+Tn=(1*1+2+n*n)*n+(2+4+6+...+2n)

==>Tn=[n(1+n)^2+n*n+n]/2

带入n=10 ==>T10=660