(1+3+5+7+…+299)-(2+4+6+…+298)= 1 + (3-2) + (45-4) + ...... + (299-298)= 1+(1+1+......+1 ) = 1+149 =150 321-1-3-5-7-9-11-13-15-17-19 = 312 - 10 * (1+19)/2 = 312 - 100 = 212 1+2+3+...+50= 50 * (1+50) = 5050