Question: Factorise:
a2 + 6a − 91
Solution:
The given expression is $a^{2}+6 a-91$.
Find two numbers that follow the conditions given below :
Sum $=6$
Product $=-91$
Clearly, the numbers are 13 and $-7$.
$a^{2}+6 a-91=a^{2}+13 a-7 a-91$
$=a(a+13)-7(a+13)$
$=(a+13)(a-7)$
