Answer:
Step-by-step explanation:
REcall that the triangular numbers are obtained by summing up the integers up to x. For example, the value of the function at x=2 is 1+2 = 3. At x=3 it is 1+2+3 = 6. So, at x we should add the first x integers. There is a formula for that, which is [tex]\sum_{i=1}^{x} i = \frac{x(x+1)}{2}[/tex] which is equivalent to 0.5x^2+0.5x
So none of the options is the answer
