Question

设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5S6 +15=0.1.若S5=5,求S

Answer 1

由S5S6 +15=0. 1,S5=5得

5S6=-14.9,

S6=-2.98,

∴a6=S6-S5=-7.98,

S6=3(a1+a6)=-2.98,

a1+a6=-2.98/3,

a1=7.98-2.98/3=2096/300=524/75.

2.d=(a6-a1)/5=-449/150.