Question

一道数学问题,要详解.已知等差数列-16,-12,-8…前n项和为72,求n的值.

Answer 1

第n项是-16+4*(n-1)=4n-20

(-16+4n-20)*n/2=72

4n^2-36n-144=0

n^2-9n-36=0

(n-12)(n+3)=0

因为n>0

所以n=12