(1)∑(n=1→50)2n(2)原式=1²-1+2²-1+3²-1+4²-1+5²-1=50(3)原式=-1/(1*2)-1/(2*3)-...-1/(2017*2018)=-(1-1/2+1/2-1/3+...+1/2017-1/2018)=-1+1/2018=-2017/2018
