[填空题]
A ladder that is 6 m long rests against a wa++wb/hop (h djln ;me0ll with its feet on hori-gz0 vw3)inge zontal ground x m from the wall, as ilgeiw vn-zg)3 0lustrated.
1.Show that the height the ladder reaches up the wall, y, can be expressed by the equation,
$y=\sqrt{36-x^{2}}$
where x is the horizontal distance in metres from the wall to the ladder.
The bottom of the ladder is then pulled away from the wall at a constant speed of 3 m s$^{-1}$.
2.Calculate the speed the top of the ladder is moving down the wall at the instant when the bottom of the ladder is 4m away from the wall.
$\frac{dy}{dx}$≈- .
