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Usually we describe proofs in algebra by algebraic means, I think it may be useful to introduce geometrical approaches to those proofs to improve creativity skills of students, what are the examples we can use in this regard and from where we can find more suited for advanced level students?
Here are few issues I have in my mind,

  1. sum of two consecutive triangle numbers is a square number.
  2. sum of cubes of first $n$ positive integers is square of sum of first $n$ positive integers.
  3. ${n+1 \choose 2} = \frac{n \cdot (n+1)}{2}$
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    $\begingroup$ Are you asking about the importance of geometrical approaches in teaching algebra, or about examples for more advanced level pupils/students? These are different questions and should be posed as such. $\endgroup$
    – Tommi
    Mar 17 at 7:25
  • $\begingroup$ @Tommi thanks for sharing your views, I personally think it may be important but need to know common view about it and what other cases can be used and how to find more examples? $\endgroup$
    – Janaka Rodrigo
    Mar 17 at 7:41
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    $\begingroup$ I recommend editing the question to be a bit more clear and targeted towards one of these purposes. $\endgroup$
    – Tommi
    Mar 18 at 11:42
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    $\begingroup$ I use a rectangle divided by a horizontal and a vertical line to break down a difference of products ($ab-cd$, $f(x)g(x)-f(x)g(x)$, etc.). That is not how I was taught, though. I was taught to add and subtract a clever term. I think of it as mnemonic device: I know it produces a correct algebraic result (I've proved it before), but the image is easier to remember. I don't know if that's the sort of thing you're looking for. $\endgroup$
    – Raciquel
    Mar 18 at 20:00
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    $\begingroup$ Yet another example may be proving irrationality of square root 2 by geometry. $\endgroup$
    – Janaka Rodrigo
    Mar 19 at 17:15

3 Answers 3

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I offer this (community wiki) only to illustrate the OP's 2nd example.

From the Archimedes Lab Project:

SumOfCubes

Quite beautiful!

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    $\begingroup$ Thanks for your contribution to give better picture of the issue $\endgroup$
    – Janaka Rodrigo
    Mar 19 at 8:46
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    $\begingroup$ I love this example. $\endgroup$
    – Sue VanHattum ♦
    Mar 20 at 2:27
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This may not be what you have in mind, but your question reminds me of “proofs without words” aka visual proofs. The three examples you gave have relatively famous visual proofs.

There’s some contention over whether visual proofs should be considered proofs, as this article explains. Essentially, since proofs without words do not make the logic of their arguments explicit, some might argue that they cannot be considered proofs. The opposing viewpoint is that

If a [proof without words] presents a mathematical idea with sufficient apparent evidentiary force to convince a mathematician of the truth of a statement, why count it as anything other than a proof?

The linked article explores this debate in greater detail.

As for the pedagogical value of visual proofs, I think they are beneficial if used in moderation. They are concrete examples of the mathematical notion of “elegance,” and in this way can help students to understand the culture of modern mathematics. They also embody the idea that a different representation can make a difficult problem more tractable, and this is a valuable lesson for students. It can also be a useful exercise to have students “fill in the words” to a proof without words.

However, many proofs without words of algebraic statements are not very generalizable, and I think that the priority should be having students become proficient with basic proof techniques. In any discipline, technique is important for creativity.

Roger B. Nelsen has put together three volumes of proofs without words, many of them algebra-related. These could be a good source of exercises for your students.

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Here's a well-known one:

$$\sum_{n=1}^{\infty} \frac{1}{2^n} = 1$$

enter image description here

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