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gave credit to dennis

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edited May 22, 2017 at 22:00

05AB1E, 77 6 bytes

EDIT: Thanks to @Dennis: Saved a byte because I forgot about the increment operator

8*1+t8*>t.ï

n is triangular if sqrt(8n + 1) is an integer

How it works

8* # multiply implicit input by 8
  1+> # add one
    t # sqrt
     .ï # is integer

8*1+t.ï

n is triangular if sqrt(8n + 1) is an integer

How it works

8* # multiply implicit input by 8
  1+ # add one
    t # sqrt
     .ï # is integer

EDIT: Thanks to @Dennis: Saved a byte because I forgot about the increment operator

8*>t.ï

n is triangular if sqrt(8n + 1) is an integer

How it works

8* # multiply implicit input by 8
  > # add one
   t # sqrt
    .ï # is integer

Source Link

created May 22, 2017 at 21:47

8*1+t.ï

n is triangular if sqrt(8n + 1) is an integer

How it works

8* # multiply implicit input by 8
  1+ # add one
    t # sqrt
     .ï # is integer