# formula for number triangles

Hi, I have a triangle starting from $0$ and going up by one on the bottom row until there are $r$ items on the bottom row and there are $r$ rows a number is formed by adding the two numbers towards the fat end of the triangle together.

Example: $$x \downarrow\$$ $$\begin{matrix}12\end{matrix}\\ \begin{matrix}4 & 8\end{matrix}\\ \begin{matrix}1 & 3 & 5\end{matrix}\\ \begin{matrix}0 & 1 & 2 & \boxed{3}\end{matrix}$$ $$\begin{matrix}& & r - 1 \uparrow\end{matrix}$$ $\therefore$ when $r = 4$, $x = 12$.

When $r = 8$ $$\begin{matrix}0\ \ & 1\ \ & 2\ \ & 3\ \ & 4\ \ & 5\ \ & 6\ \ & 7\ \ \end{matrix}\\ \begin{matrix}1\ \ & 3\ \ & 5\ \ & 7\ \ & 9\ \ & 11\ & 13\ \end{matrix}\\ \begin{matrix}4\ \ & 8\ \ & 12\ & 16\ & 20\ & 24\ \end{matrix}\\ \begin{matrix}12\ & 20\ & 28\ & 36\ & 44\ \end{matrix}\\ \begin{matrix}32\ & 48\ & 64\ & 80\ \end{matrix}\\ \begin{matrix}80\ & 112 & 144 \end{matrix}\\ \begin{matrix}192 & 256 \end{matrix}\\ \begin{matrix}448\end{matrix}\\$$ $\therefore x = 448$

How do I work out $x$ from $r$? What is the formula? Any help by suggesting a new viewpoint or formula(e) would be greatly appreciated. Thank you.

A recursive solution: number the rows $0$ to $r-1$ with $0$ being the fattest, notice that the difference between consecutive numbers in row $k$ is $2^k$.

Denote by $f(n)$ the number in the last row of a triangle with $n$ rows. Then $f(n)=2f(n-1)+2^{n-2}$

Notice $f(1)=0,f(2)=1,f(3)=4$

From this recurrence we pass to $f(n)=(n-1)2^{n-2}$

The proof is by induction. Suppose $f(n)=(n-1)2^{n-2}$

then $f(n+1)=(n-1)2^{n-1}+2^{n-1}=n(2^{n-1})$ as desired.

• but when n = 4 f(n) = (4-1)2^4-1 = 3x2^3 = 3*8 = 24 – user181782 Jun 9 '14 at 16:43
• oh, yes give me a second. – Jorge Fernández Hidalgo Jun 9 '14 at 16:45
• i thoght it was 12 as r1 = 0,1,2,3 r2=1,3,4 r3=4,8 r4=12 – user181782 Jun 9 '14 at 16:48
• The formula is $(n-1)2^{n-2}$ – Jorge Fernández Hidalgo Jun 9 '14 at 16:48
• I had made a mistake, but this one is correct. – Jorge Fernández Hidalgo Jun 9 '14 at 16:50

Consider the first two 'columns'. Notice any patterns?

$$0 \space 1\\1 \space 3 \\4 \space 8\\ 12\space 20$$

What is the rule?

$$a_{i+1} = a_i + b_i$$ $$b_{i+1} = a_{i+1} + 2 (b_i - a_i) = 3b_i - a_i$$

Try find a formula for $a_n$.

• what is the Ai and Bi - what does it mean – user181782 Jun 9 '14 at 16:36
• Sorry, $a_i$ refers to the first entry of the $i^{th}$ column and $b_i$ the second entry. By column, I mean reading the two diagonal columns of the triangle – muzzlator Jun 9 '14 at 16:41