求极限:lim(x→0)[cosx+cos^x+cos3(次方)x+……+cosn(次方)x] /(cosx-1),[n(n+1)]/2,

问题描述:

求极限:lim(x→0)[cosx+cos^x+cos3(次方)x+……+cosn(次方)x] /(cosx-1),[n(n+1)]/2,

=lim[-sinx-2cosxsinx-3cos^2xsinx-…-ncos^(n-1)xsinx]/(-sinx)
=lim[1+2cosx+…ncos^(n-1)x
=1+2+…n
=(1+n)n/2谢谢,但还没学到导数,应该是用其他方法算