For a light that has an intensity of 60 candles at its source, the intensity in candles, S, of the light at a point d feet from the source is given by the formula

s=~$60k \over d^2$~ where k is a constant. If the intensity of the light is 30 candles at a distance of 2 feet from the source, what is the intensity of the light at a distance of 20 feet from the source?


~$3 \over 10$~ candle

~$1 \over 2$~ candle

1 candle

2 candles

3 candles

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题目分析：

先求常数 K,根据题目中给的条件,S=30, d=2;带入公式求 k

30=~$\frac{60*k}{4}$~;解出 K=2;

K=2,d=20 求出相应的 S；

~$\frac{60*2}{20*20}=\frac{3}{10}$~

S= ~$\frac{3}{10}$~ .

综上:答案就是A.

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