(10) A ladder (梯）10 feet long rests against a vertical wall (斜靠於一垂直牆面).of the bottom (底）of the ladder slides away (滑開）from the wall at a rate of 1 fl/sec, how fast is the top of the ladder sliding down the wall when the bottom of the ladder is 6ft from the wall?

依靠在垂直牆高為 y，梯腳距牆腳xx2+...