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有趣的代数

An Interesting Algebra Problem:


If x + 1/x = 7, then x2 + 1/x2 = ?

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上期答案:
Answer: (e)
Solution: Gold concentration = weight of gold/total weight
0.80 = (20 • 0.60 + x)/( 20 + x)
12 + x = 16 + 0.8x
0.2x = 4
x = 20 grams