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Be Reasonable

Prove that sqrt2, sqrt3 and sqrt5 cannot be terms of ANY arithmetic progression.

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Janusz Asked

In y = ax +b when are a, -b/a, b in arithmetic progression. The polynomial y = ax^2 + bx + c has roots r1 and r2. Can a, r1, b, r2 and c be in arithmetic progression?

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Summats Clear

Find the sum, f(n), of the first n terms of the sequence: 0, 1, 1, 2, 2, 3, 3........p, p, p +1, p + 1,..... Prove that f(a + b) - f(a - b) = ab.

Polite Numbers

Stage: 5 Challenge Level: Challenge Level:1

Why do this problem?

This problem provides an interesting situation in which to consider and practise the formula for the sum of an arithmetic progression with common difference $1$ (although the knowledge of APs is not necessary). Relating the sums to a visual representation on a number line will reinforce the meaning of the algebraic process involved in the derivation of the AP formula.

Possible approach

Ask students to start the problem by finding the APs leading to $544$ and $424$. Once they have represented their sums on the number line, can they explain in words why the sums will yield $544$ and $424$ without calculation? They can use the AP formula to check their answers (they will need calculators).

Next students can look at the numbers $1000$ and $1001$. Again, can they explain in words using a representation on a number line why their answers work? They can use the AP formula to check that they are indeed correct.

In searching for the APs, students should realise that the factorisation of a number is important in breaking it down into an AP. They should realise or be encouraged to relate the factors of a number to the terms $a$ and $n$ of the arithmetical progression. You might suggest that students also try to find APs for other numbers such as $40, 246$ and $500$.

Finally, note that there are two possible ways of writing the formula for the sum of an AP
$$S_n = \frac{n}{2}\left[2a+(n-1)\right] = n\left[a+\frac{(n-1)}{2}\right]$$
In words, what are the equations saying? Which of these two representations would be useful for different polite numbers?

In order to prove the case that a particular number is polite, students will need to show that it can be written in one of the above algebraic forms: essentially, they need to try to break down each number into two appropriate factors.

Proving the final part will first require a conjecture on the part of the students. Before attempting to prove their result they might be advised to test out their conjecture on some small numbers less than $40$. This conjecturing is an important part of the mathematical process, irrespective of whether or not a final proof is constructed.

Key questions

Can you explain in words why your consecutive sum will yield the required answer? Could you see how this approach might work for other numbers?

Why is the factorisation of the numbers important?

What is your conjecture for which numbers are polite?

Possible extension

Which numbers can be written as the sum of an arithmetical progression with common difference $2$?

Possible support

Some students may find it hard to find the APs leading to $544$ and $424$ straight away, and may benefit from experimenting with smaller numbers, perhaps using the approaches suggested in Consecutive Sums . This should help them to spot some patterns and make some conjectures, which can then be investigated using the AP formula.