350+ (350+81x1) +(350+81x2)+(350+81x3)+````` (350+81xN) =350(N+1)+81(1+2+3+……+N) =350(N+1)+81N(N+1)/2 =(N+1)(350+81N/2) =177500 得(N+1)(700+81N)=355000 81N²+781N-354300=0 根据题意,N应当是自然数,而这个方程的解不是自然数,所以建议重新检查一下题目!