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设a,b,c,d∈R,若a,1,b成等比数列,且c,1,d 成等差数列,则下列不...
设a,b,c,d∈R,若a,1,b成等比数列,且c,1,d 成等差数列,则下列不等式恒成立的是( )
A.a+b≤2cd
B.a+b≥2cd
C.|a+b|≤2cd
D.|a+b|≥2cd
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