✔ 最佳答案
x=1(2)+2(2^2)+3(2^3)+...+n(2^n)
求以n表x
Sol
x=1*2+2*2^2+3*2^3+4*2^4+…….+n*2^n
2x= 1*2^2+2*2^3+3*2^4+…….+(n-1)*2^n+n*2^(n+1)
x-2x=1*2+1*2^2+1*2^3+…..+1*2^n-n*2^(n+1)
=(2+2^2+2^3+…..+2^n)-n*2^(n+1)
=(2+2+2^2+2^3+…..+2^n-2)-n*2^(n+1)
=(2^2+2^2+2^3+…..+2^n-2)-n*2^(n+1)
=(2^3+2^3+…..+2^n-2)-n*2^(n+1)
=[2^(n+1)-2]-n*2^(n+1)
=(1-n)*2^(n+1)-2
x=(n-1)*2^(n+1)+2