Question

数列｛an｝中①Sn=π（2n²+n）/12 ②Sn=2/3（3n-1） 求an

Answer 1

(1)

Sn=π(2n^2+n)/12 (1)

S(n-1)=π(2(n-1)^2+(n-1))/12 (2)

(1)-(2)

an=(π/12)(4n-1)

(2)

Sn=(2/3)(3n-1) (1)

S(n-1)=(2/3)(3(n-1)-1) (2)

(1)-(2)

an =2