Question

已知数列{an}是公差不为零的等差数列,a1＝1且a1 a2 a4成等比数列,1求通项an 2令bn＝an+2^an求数

Answer 1

(1) 设a2=a1+d,a4=a1+3d因为a1＝1且a1 a2 a4成等比数列,所以a1 *a4=a1^2即1*(1+3d)=(1+d)^2解得d=1或0（舍去）所以an=a1+(n-1)d=1+(n-1)*1=n(2)bn＝an+2^an=n+2^n,则b1=3用分组求和的方法,b1+b2+.+bn=n(n+1)/2+b1*(1...