`a)1+2+3+...+x=19900`
`x.(x+1):2=19900`
`x.(x+1)=19900.2`
`x.(x+1)=39800`
`x.(x+1)=199.200`
`=>x = 199`
`b)1+3+5+...+x=51^2`
`1+3+5 + ... + x = 2601`
`( x + 1 ).[(x-1):2+1] : 2 = 2601`
`( x+1 ).(x-1):2+1 = 2601.2`
`( x+1 ).(x-1):2+1 = 5202`
`( x+1 ).(x-1):2 =5202-1`
`(x+1).(x-1):2=5201`
`( x+1 ).(x-1)=5201.2`
`( x+1 ).(x-1)=10402`
`=>( x+1 ).(x-1)=` `không` `có` `giá` `trị`
`=>x=emptyset`
