Cho a+b+c+d=2. Chứng minh a^2+b^2+c^2+d^2>=1 Ghi rõ ràng ra nhé
Question
Cho a+b+c+d=2. Chứng minh a^2+b^2+c^2+d^2>=1
Ghi rõ ràng ra nhé
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