Question

1*2+2*3+3*4.79*80

Answer 1

1*2+2*3+3*4+.+79*80

=(1*2*(3-0) +2*3*(4-1)+3*4*(5-2)+.+79*80*(81-78))÷3

=(1*2*3+(2*3*4-1*2*3)+(3*4*5-2*3*4)+.+(79*80*81-78*79*80))÷3

=79*80*81÷3

=170640