=(1+100)+(2+99)...+(50+51)=101*50=5050
1+99+ 2+98+ 3+97......+49+51 +50100×49+50+100=5050
=(1+100)×100÷2=101×100÷2=10100÷2=5050
=（1+100）*100/2=101*50=5050
