Question

在等差数列中①a2+a3+a8=9,a3a5a1= - 21,求an ② ap=m,aq=n,求ap+q,③a15=10

Answer 1

② ap=m,aq=n,求ap+q

答：ap=m=a1+（p-1）d,aq=n=a1+（q-1）d

m-n=（p-q）d ,d=m-n/p-q

ap+q=a1+（p+q-1）d=m+q*(m-n/p-q)

③a15=10,a45=90,求a60

答：an为等差数列,公差为d,则a15n是公差为15d的等差数列；

a15=10,a45=90,a30=50

a60=130.

10 50 90 130

④a5=10,a12=31,求an,d

答：a5=a1+4d=10,a12=a1+11d=31

a1=-2 d=3 an=-2+(n-1)*3=3n-5

确认一下第一题的题目是否有误