原式=(1-2)+ (2-3)+…+(19-20)=-1×19=-19或者原式=(1+2+…+19)-(2+3+…+20)=19×(1+19)/2-19 ×(2+20)/2=19×10-19×11=19×(10-11)=-19
-2 +2 抵消，类推-19 +19抵消=1-20=-19
负19
