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    Question

    If x = 555, y = 554, z = 556, then the value of

    x2 + y2 + z2 - xy-yz-zx  
    A 2 Correct Answer Incorrect Answer
    B 3 Correct Answer Incorrect Answer
    C 4 Correct Answer Incorrect Answer
    D 5 Correct Answer Incorrect Answer
    E none of these Correct Answer Incorrect Answer

    Solution

    x2 + y2 + z2 - xy-yz-zx = 1/2 {(x-y)2+(y-z)2+(z-x)2 }  = 1/2 {(555-554)2+(554-556)2+(556-555)2 } = 1/2 {(1)2+(-2)2+(1)2 } = 1/2 (1 + 4 + 1) = 1/2 × 6 = 3

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