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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 2.1 m/s, how fast is the length of his shadow on the building decreasing when he is 4 m from the building?

Answer 1

once you have that relation, then you can replace the variables by the numbers given. by similar triangles that relation is

y12=2x then differentiate both sides