a = (-2013*2013-2013)/(2012*2012+2012) = -(2013*2013+2013)/(2012*2012+2012)
b = (-2014*2014-2014)/(2013*2013+2013) = -(2014*2014+2014)/(2013*2013+2013)
c = (-2015*2015-2015)/(2014*2014+2014) = -(2015*2015+2015)/(2014*2014+2014)
abc
= -(2015*2015+2015)/(2012*2012+2012)
= -[2015*(2015+1)]/[2012*(2012+1)]
= -(2015*2016)/(2012*2013)
= -(2015*168*4*3)/(4*503*671*3)
= -(2015*168)/(503*671)
= -338520/337513