2x^2 -3x^3 5x^4 -9x^5 17x^6 ,33x^7,.
第n个单项式是:
(-1)^(n-1)*(2+1+2+...+2^(n-2))*x^(n+1)
=(-1)^(n-1)*(2^(n-1)+1)*x^(n+1)