等差数列{an}中,a5=11,a8=5,求an

问题描述:

等差数列{an}中,a5=11,a8=5,求an

∵a8-a5=3d
∴d=-2
∵a5=a1+4d
∴a1=19
∴an=a1+(n-1)d
=18-2(n-1)
=21-2n