Answer :
His shadow is shortening at a rate of dy/dt = 0.6m/s at the time.
What is triangle total degrees?
The triangle's three angles add up to a total of 180 degrees
The individual is standing at point D and has his head at point E at the time indicated in the problem.
At that moment, his shadow is visible on the wall at y = BC.
The two right triangles ABC and ADE both contain similar triangles. The ratios on their respective sides in the similar triangles are equal:
AD/AB = DE/BC
8/12 = 2/y
y = 3
If we suppose that the man is x distance from the building, then the distance from the spotlight to the man is 12 x.
12-x/12 = 2/y
1-1/12x = 2 1/y
Let's take derivatives of both sides:
-1/12dx = -2 .1/y²dy
Divide both sides by dt as follows.
-1/12.dx/dt = 2/y²dy/dt
At the time specified:
dx/dy = 1.6m/s
y = 3
Let's connect them
- 1/2 (1.6) = -2.9 dy/dt
dy/dt = 1.6/12×9/2 = 0.6m/s.
At the time in question, this is the pace at which his shadow's length is shortening.
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The complete question is:
"A spotlight on the ground shines on a wall 12 m away. If a man 2 m tall walks from the spotlight toward the building at a speed of 1.6 m/s, how fast is the length of his shadow on the building decreasing when he is 4 m from the building?"