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    Question

    if a2 + b2 +

    c2 = 2(4a -5b -6c)-77 , then a-b-c = ?
    A 14 Correct Answer Incorrect Answer
    B 15 Correct Answer Incorrect Answer
    C 10 Correct Answer Incorrect Answer
    D 11 Correct Answer Incorrect Answer

    Solution

    a2 + b2 + c2 = 2(4a -5b -6c)-77 So a2 + b2 + c2 -2×4a +2 ×5b + 2×6c +77 = 0 a2 -2×4a +16 + b2 +2 ×5b+25 +c2 +2×6c+36 = 0 So (a-4)2 +(b+5)2+(c+6)2 = 0 As we know that if x2 + y2 +z2 = 0 then x=y=z = 0 So here a-4 = 0 so a =4; b +5 = 0 so b = -5 And c+6 =0 so c = -6 So now a-b-c = 4-(-5)-(-6) = 4+5+6 = 15

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