3 x 3^2n+1-21 x 9^n
问题描述：
3 x 3^2n+1-21 x 9^n
求价
答
3 x 3^2n+1-21 x 9^n
=3 x 3^2n+1-7 x3 x3^2n
=3 x 3^2n+1-7 x3^2n +1
=3^2n +1(3-7)
=-4x 3^2n +1