This sequence has generating function
(if we include 
for a moment)
Recall that for 
, we have
Take the derivative to get
Take the derivative again:
Take the derivative one more time:
so we have
To tessellate a surface using a regular polygon, the interior angle must be a sub-multiple (i.e. factor) of 360 degrees to cover completely the surface.
For a regular three-sided polygon, the interior angle is (180-360/3)=60 °
Since 6*60=360, so a regular three-sided polygon (equilateral triangle) tessellates.
For a regular four-sided polygon, the interior angle is (180-360/4)=90 °
Since 4*90=360, so a regular four-sided polygon (square) tessellates.
For a regular five-sided polygon, the interior angle is (180-360/5)=108 °
Since 360/108=3.33... (not an integer), so a regular five-sided polygon (pentagon) does NOT tessellate.
For a regular six-sided polygon, the interior angle is (180-360/6)=120 °
Since 3*120=360, so a regular six-sided polygon (hexagon) tessellates.
Http://www.icoachmath.com/math_dictionary/equivalent_equations.html
Here's a website that might help
15x^2 = 12x
divide each side by x
15x = 12
now divide each side by 15
x = 12/15 or 4/5 which is also 0.8
