Question

Which of the following numbers cannot be represented as a triangle using dots?

• A. 10
• B. 15
• C. 3
• D. 12

Answer 1

The correct option is D 12

We know that triangular numbers are formed by taking n as the number of dots in the base and proceeding upwards by reducing one dot each until we get only one dot where n = 2,3,4,5 ...
From the above figure we can see that by taking n = 2 we can represent 3, n = 4 we can represent 10 and n = 5 we can represent 15.

Therefore, we cannot represent 12 as the triangle would be incomplete.