Take a point (1,2) and a nearby point (1 + △ x, 2 + △ y) on the image of curve y = x2 + 1, then △ y △ x is______ .

Take a point (1,2) and a nearby point (1 + △ x, 2 + △ y) on the image of curve y = x2 + 1, then △ y △ x is______ .

If △ y △ x = (1 + △ x) 2 + 1 − (1 + 1) △ x = △ x + 2, then △ y △ x is △ x + 2