The nth triangular number is n(n+1)/2
820
40th = fortieth.
Fortieth.
this how you write it fortieth .
The ordinal number 40th is spelled fortieth.
The ordinal numeral 40th is spelled "fortieth."
40 is forty. 40th is fortieth.
the fortieth largest state is South Carolina
The cardinal number 40 is spelled "forty" and the adjective (ordinal) 40th is "fortieth".
Fortieth Army - Japan - was created in 1945.
Fortieth Army - Japan - ended in 1945.
The Fortieth Door was created on 1924-05-25.
Glorious rhymes with fortieth, there are many more on google if you look it up.