Question

知道和、公差、首项,如何求末项

Answer 1

Sn=(a1+an)n/2

an=a1+(n-1)d

n=(an-a1)/d+1

所以Sn=(a1+an)[(an-a1)/d+1]/2

[(an)^2-(a1)^2]/d+a1+an=2Sn

(an)^2+dan-(a1)^2+da1-2dSn=0

解这个方程即可