Question: Factorise:
y2 + y − 72
Solution:
The given expression is $y^{2}+y-72$.
Find two numbers that follow the conditions given below:
Sum $=1$
Product $=-72$
Clearly, the numbers are 9 and $-8$.
$y^{2}+y-72=y^{2}+9 y-8 y-72$
$=y(y+9)-8(y+9)$
$=(y+9)(y-8)$
