☆Trick☆

1+2+3+4+5.....99+100+99 +....5+4+3+2+1=10000

When sch type of question asked 
Do a square of big number
Here ( 100)^2=10000

1+2+3..........+11+10+....3+2+1=121
11 is big number ..so 11^2=121
Like that...so on.. 

this technique come from this formulas i explain u
(1+11)*11/2 +(10+1)10/2
=11*6 +11*5
=11*11
=121 hence prooved