2018 DHS Promo Q3

Solved by

Timothy Gan

This is Tim. Tim loves to teach math. Tim seeks to improve his teaching incessantly! Help Tim by telling him how he can do better.
2018 DHS Promo Q3 

It is given that $\text{f}\left( r \right)=2{{r}^{3}}-3{{r}^{2}}+r-4$.

(i)

Show that $\text{f}\left( r+1 \right)-\text{f}\left( r \right)=k{{r}^{2}}$, where $k$ is a constant to be determined.

[2]

(ii)

Deduce a formula for $\sum\limits_{r=1}^{n}{{{r}^{2}}}$, fully factorising your answer.

[3]

(iii)

Hence show that $\sum\limits_{r=4}^{n-3}{{{\left( r-1 \right)}^{2}}=\frac{1}{6}\text{g}\left( n \right)-5}$, where $\text{g}\left( n \right)$ is to be determined.

[3]

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Published: 21st March 2024
2018 DHS Promo Q3
Written by
Timothy Gan
This is Tim. Tim loves to teach math. Tim seeks to improve his teaching incessantly! Help Tim by telling him how he can do better.
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Published: 21st March 2024

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