a) 25x^2-2xy+1/25y^2 khi x=-1/25 và y=-5 b) x^2+y^2 biết x+y=3 và xy = -8 c) x^2+y^2 biết x-y=3 và xy=-2 Tính giá trị ( Ưu tiên sử dụng HĐT)

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