conflict in "Acceleration: Airbus A380 take off distance."

Dear Khan,

In the calculation of how long the runway will have to be for the plane to take off, you mentioned that we use average velocity (0 + 78)/2 times the time 78sec, we get 3042m which is the answer.  However, if I use another way: 1+2+3+.....+78=3081m.  Why there is such discrepancy?  Shouldn't the two ways get the same answer?  If I calculate the average velocity using (1+78)/2, then times 78sec, I get 3081m.   So could you explain what's wrong?  Thanks!

Best regards,

Wang Chen