Pascal's triangle

The Pascal's triangle is a convenient tabular presentation for the binomial coefficients. Already known in the 11th century, it was adopted in Western world under this name after Blaise Pascal published his Traité du triangle arithmétique ("Treatise on the Arithmetical Triangle") in 1654.

For instance, we can use Pascal's triangle to compute the coefficients of

$(x+y)^{5}=1\times a^{4}+4\times a^{3}b+6\times a^{2}b^{2}+4\times ab^{3}+1\times b^{4}~$

The coefficients 1, 4, 6, 4, 1 appear directly in the triangle.

${\begin{matrix}&&&&&1\\&&&&1&&1\\&&&1&&2&&1\\&&1&&3&&3&&1\\&1&&4&&6&&4&&1\\&&&&&\cdots \end{matrix}}$

Each coefficient in the triangle is the sum of the two coefficients over it. For instance, $6=3+3$. The binomial coefficients relate to this construction by Pascal's rule, which states that if

${n \choose k}={\frac {n!}{k!(n-k)!}}$

is the kth binomial coefficient in the binomial expansion of (x+y)ⁿ, then

${n \choose k}={n-1 \choose k-1}+{n-1 \choose k}$

for any nonnegative integer n and any integer k between 0 and n.

Those coefficients have applications in algebra and in probabilities. From the triangle, we can equally compute the Fibonacci numbers and create the Sierpinski triangle. Isaac Newton, after studying it, found new methods to extract the square root and to calculate natural logarithms of a number.