Question

等差数列{An}中,A6=5,A3+A4=5,求A9,S9.

Answer 1

假设公差为d

A6=A3+3*d=5

A3+A4=2*A3+d=5

解之得：A3= 2,d=1

A9=A3+6*d=8

S9=9*(0+8)/2=36