(1)题错了.既然sn=-5,说明d不应为正值,应是负值,d=-1/6才对.
(1)a1=5/6,d=-1/6 sn=-5
-5=5n/6+n(n-1)×(-1/6)/2化简后得:n^2-11n-60=0解n=15或n=-4(无意义)
an=a15=a1+14d=-3/2
即:n=15 a15=-3/2
(2)s3=a1+a2+a3=a1+a1*q+a1*q^2=2+2q+2q^2=26化简:q^2+q-12=0解得q=3或q=-4(无意义)
a3=a1*q^2=2*3^2=18
即:q=3 a3=18
(3)a3=3/2 a1*q^2=3/2 ①
s3=a1+a2+a3=a1+a1*q+3/2=9/2化简a1+a1*q=3 ②
解方程组:q=1或q=-1/2(无意义) a1=3/2
即:a1=3/2 q=1