Question

设等差数列{an}的前n项和为Sn，若a5=a3?∫20(2x+12)dx，则S9S5=（　　）A．9B．259C．2D．92

设等差数列{an}的前n项和为Sn，若a5=a3?∫20(2x+12)dx，则S9S5=（　　）A．9B．259C．2D．925

Answer 1

∵

∫

2 0

(2x+

1

2

)dx=[x2+

1

2

x]|₀²=5，
∴a₅=5a₃，
∴

S9

S5

=

9 2 ×2a5

5 2 ×2a3

=

9

5

×

a5

a3

=

9

5

×

5a3

a3

=9．
故选A．