In a perfect square trinomial , the terms are in the form of
[tex] a^2 + 2ab + b^2 [/tex] , so that it can be factored and we get [tex] (a+b)^2 [/tex]
Now lets compare the given expression
[tex] x^2 - 10x + n [/tex]
Now we can compare this with [tex] a^2 + 2ab + b^2 [/tex]
we get a=x , 2ab = -10x
So we get
2(x) b = -10x
it means 2b = -10
b=-5
But [tex] n=b^2 = (-5)^2 = 25 [/tex]
Hence, n= 25