Câu hỏi 5Nếu $f(n)=\dfrac13n(n+1)(n+2),$ thì $f(r)-f(r-1)$ bằng:$r(r+1)$$(r+1)(r+2)$$\dfrac13r(r+1)$$\dfrac13(r+1)(r+2)$$\dfrac13r(r+1)(r+2)$