Find the rule 1 + 2 + 3 +. 49 + 50 + 49 +. 3 + 2 + 1=

Find the rule 1 + 2 + 3 +. 49 + 50 + 49 +. 3 + 2 + 1=

1+2+3+.49+50+49+.3+2+1=1+2+3+.49+50+50+49+.3+2+1-50=2*(1+2+3+.49+50)-50=2*(1+50)*50/2-50=2550-50=2500