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In a right triangle, the hypotenuse is of fixed length of 15 units, one side is increasing in length by 4 units per second while the third side is decreasing in size. At a certain instant the increasing side is of length 9 units. Find the rate of change of the third side at the same instant.

Multiple choice answers
a. -3 units per second
b. -5/2 units per second
c. -3/2 units per second
d. -5/4 units per second
e. -3/4 units per second
which on is it? I got -51/8 units per second but that's not right

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1 answer
  1. we have

    x^2+y^2 = 15^2
    x dx/dt + y dy/dt = 0

    Let x be decreasing, and y increasing. Then when y=9, x=12, and we have

    12 dx/dt + 9*4 = 0
    dx/dt = -3

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