1+2.......1000=? Plz come in!

As you know :
1 + 1000 = 1001
2 + 999 = 1001
3 + 998 = 1001
如此類推
500 + 501 = 1001

1+2........+1000, 共有 500 個這樣的組合 (1000 除 2)

So, 1+2+3+.........1000

= 1000 (1+1000) / 2

= 500 500

這是一題三角型數
``1+1000
------------- x 1000
`````2

=`1001x1000
``-----------------
`````````2

=1001x500

=500500

(1+1000)x1000/2
=1001x500
=500500

公式：(頭項+尾項)x項數/2

1+2.......1000
= (1+1000)*1000/2
=1001*1000/2
=500500

(1 + 1000) * 1000 / 2 = 500500

Ans 5050

=500500

1+2.....1000
= (1 + 1000) + 92 + 999) + (3 + 998) + ... + (500 + 501) ...................(500 terms)
= 1001 + 1001 + 1001 + ... + 1001 .......................(500 terms)
= 1001 x 500
= 500,500

(1+1000)*1000/2=500500
[(1st term+last term)*no. of terms/2]

1+999+2+998+3+997+4+996+5+995.......1000
= 500x1000
= 500,000

Answer = (1 + 1000) * 1000 / 2 = 500500