A man is walking at the rate of 6.5 km/hr towards the foot of a tower 120m high. At what rate is he approaching the top of the tower when he is 50m away from the tower.
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dx/dt = 6.5 km/ hr
dy/dt = ?
x² +120² = y²
2x dx/dt = 2y dy/dt
xdx/dt = ydy/dt
x(6.5) = y dy/dt
dy/dt = 6.5x/y --- (1)
When x = 50 then y² = 50² + 120²
= 25000 + 14400
y² = 16900
y = 130
We will now put the value x = 50 and y = 130 in the equation 1
dy/dt = 6.5 x 50/130 = 2.5
dy/dt = 2.5km/hr
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