If x + a is a common factor of expressions f(x)=x2+px+q and g(x)=x2+mx+n; show that : a=n−qm−p
Since x+a is the factor of both the equations so we can say
F(-a)=0
and
G(-a)=0
(−a)2 + p·(-a) + q = 0 --> a2 -ap + q = 0
(−a)2+ m·(-a) + n = 0 --> a2 -am + n = 0
Since they both equal 0, you can set them equal to each other:
a2 -ap + q = a2 -am + n
-ap + q + -am + n
am - ap = n-q
a(m-p) = n-q