xn=2ⁿp+nqn=1时2p+q=3 x1,x4,x5成等差数列,则2x4=x1+x52×(2⁴p+4q)=3+(2^5p+5q)整理,得3q=3q=1,代入2p+q=32p+1=32p=2p=1p=1 q=12.xn=2ⁿ+nSn=x1+x2+...+xn=(2+2²+...+2ⁿ)+(1+2+...+n)=2...