Question

等差数列1=1/2,s4=20,s6=?

Answer 1

a(n)=1/2+(n-1)d,n=1,2,...

S(n)=n/2+n(n-1)d/2.

20=S(4)=4/2+4*3d/2=2+6d,d=3.

S(6)=6/2+6*5d/2=3+3*5d=3+3*5*3=48.