Question

Which of the following is not a triangular number?

(a) 21
(b) 36
(c) 45
(c) 54​

Answer 1

Hi,

Answer C

Explanation:

`Every\ triangular\ number\ has\ the \ form\ (n*(n+1))/(2) \ where\ n\ is\ an\ positive\ integer.\\\\21=(42)/(2)= (6*7)/(2) \ is \ a\ triangular\ number\\\\36=(72)/(2)= (8*9)/(2) \ is \ a\ triangular\ number\\\\45=(90)/(2)= (9*10)/(2) \ is \ a\ triangular\ number\\\\55=(110)/(2)= (10*11)/(2) \ is \ a\ triangular\ number\\54 \ is\ not\ \ a\ triangular\ number`