Is RPN still relevant?

[Thomas Klemm]

(10-07-2015 08:13 PM)John Colvin Wrote:  43 + 89

It's just applying the associative law:
\[
\begin{align}
43 + 89 &= (42 + 1) + 89 \\
&= 42 + (1 + 89) \\
&= 42 + 90 \\
&= 132
\end{align}
\]

Or then you can go into the other direction:
\[
\begin{align}
43 + 89 &= 43 + (7 + 82) \\
&= (43 + 7) + 82 \\
&= 50 +82 \\
&= 132
\end{align}
\]

Why shouldn't children use this?

You can do the same with multiplication:
\[
\begin{align}
12 \times 37 &= (4 \times 3) \times 37 \\
&= 4 \times (3 \times 37) \\
&= 4 \times 111 \\
&= 444
\end{align}
\]

I just happen to know that \(111 = 3 \times 37\). So whenever I see \(37\) in a product I check if I can "borrow" \(3\) from another factor.

Cheers
Thomas