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Number Theory

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Moderate

What is $\sum_{1 \leq n \leq x} n$?

NUMTH-YKBWFC

Let $n \in \mathbb{N}$.

What is $\sum_{1 \leq n \leq x} n$? Select ALL that apply.

A

$0$ if $x < 1$

B

$1$ if $x = 1$

C

$\cfrac{\lfloor x \rfloor\left(\lfloor x \rfloor + 1 \right)}{2}$ if $x \geq 1$

D

$\cfrac{x(x + 1)}{2}$