1÷﹙2×4﹚＋1÷﹙4×6﹚＋1÷﹙6×8﹚＋···＋1÷﹙198×200﹚＝1/4{1/2+1/2*3+1/3*4+.1/99*100}=1/4{1-1/2+1/2-1/3+1/3-1/4+.1/99-1/100}=1/4*(1-1/100)=99/400