then we have a common factor in the denominator:
, provided
is the hypotenuse of the right triangle with sides
and
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
...in which elements in a row are the sum of the two elements in the row above it. These numbers are the Binomial Coefficients.
is the cube of the binomial
so the coefficients can be taken from the third row above:
1 3 3 1 and the exponents on
start with
and count down, while the exponents on
start with
and count up:
which simplifies to:
as previously.
or
or
if a product is zero one of its factors must be zero so:
either
or
or
and either
or
or
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