Express (x ^{2} - 5x + 7) (x^{2} + 5x - 7) as a difference of two squares.

Solution :

(x^{2} - 5x + 7) (x^{2} + 5x - 7)

= [x^{2} - (5 x - 7)] [x^{2} + (5 x - 7)]

= (x^{2}) ^{2} - (5x - 7)^{2}