Question

a 15-ft ladder is leaning against one wall of an alley 9 ft wide. the ladder slips, its top sliding down the wall, its foot sliding across the alley and striking the opposite wall at a speed of 4 ft/sec. How fast is the top of the ladder falling at that instance?

Answer 1

2x dx/dt +2y dy/dt = 0 
x dx/dt + y dy/dt = 0 
9*4 + 12 dy/dt = 0 
dy/dt = - 3 ft/sec