To prove that (4x - 5)^2 - 5x(3x - 8) is positive for all values of x, we can start by expanding the expression:
(4x - 5)^2 - 5x(3x - 8) = 16x^2 - 40x + 25 - 15x^2 + 40x = -14x^2 + 80x - 25
Now, we can complete the square to find that this expression is always positive:
-14x^2 + 80x - 25 = (-14x^2 + 80x) - 25
= (-14x^2 + 80x + (80/2)^2 - (80/2)^2) - 25
= (-14x + 40)^2 - 25 - 40^2 + 40^2
= 15^2 - 25
= (15 - 5)(15 + 5)
= 10(20)
= 200
Since this expression is equal to 200, which is a positive number, we can conclude that (4x - 5)^2 - 5x(3x - 8) is positive for all values of x.