f(x) = x2 - 4x + 6
f1 (x) = 2x - 4
f1 (x ) = 0 given, 2x - 4 = 0
⇒ 2x =4
⇒ x = 2
Thus f1(x ) = 0 at x = 2
Now, the point x = 2 divides the real line into two disjoint intervals namely,
(-∞,2) and (2,∞).
At (2,∞),f1 (x) > 0
⇒ f is strictly increasing at(2,∞)