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(a²+b²)(c²+d²)-(ac+bd)²=(a^2*c^2+a^2*d^2+b^2*c^2+b^2*d^2)-(a^2*c^2+2acbd+b^2*d^2)=a^2*d^2-2ad*bc+b^2*c^2=(ad-bc)^2>=0所以:(ac+bd)²≤(a²+b²)(c²+d²)...
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