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how far must a man 6 ft tall stand from a light 15ft above the ground in order to cast a shadow 20 ft long?

Sagot :

Please refer to the picture below:

This is just a problem about similar triangles. So we know that:
ΔADE ~ ΔABC
Because they share common sides then they are similar so:
[tex] \frac{AC}{AE} = \frac{BC}{DE} = \frac{15}{6} = \frac{5}{2} [/tex]
We let CE be x then this would make:
[tex] \frac{20+x}{20} = \frac{5}{2} \\ (20+x)2=5*20 \\ 40+2x=100 \\ 2x=60 \\ x=30[/tex]

Therefore the man should stand 30 feet from the light to cast a shadow of 20ft long.

View image Mlcparra16