若x+2/x=5,则x^2+4/x^2

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若x+2/x=5,则x^2+4/x^2
若x+2/x=5,则x^2+4/x^2

若x+2/x=5,则x^2+4/x^2
若x+2/x=5
两边同时平方得：
x^2+4+4/x^2=25
所以 x^2+4/x^2=21

原式 = （x+2/x）^2 - 4 = 5^2-4 = 21

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