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222-9+11+12:2*14+14 = ? ( )

Toán Lớp 7: cho (a-c)/(c-b)=a/b cmr 1/c= 1/2(1/a+1/b)

Toán Lớp 7: cho (a-c)/(c-b)=a/b cmr 1/c= 1/2(1/a+1/b)

Comments ( 1 )

  1. $\\$
    giả sử 1/c = 1/2 (1/a+1/b)
    =>1/c =1/2 (b/(ab)+a/(ab))
    =>1/c=1/2 . (a+b)/(ab)
    => 1/c = (a+b)/(2ab)
    => 2ab = ac+ bc
    => ab+ab = bc +ac
    =>ab-bc = ac-ab
    => b (a-c) = a (c-b)
    => a/b=(a-c)/(c-b) 
    Vậy nếu a/b=(a-c)/(c-b) thì 1/c=1/2 (1/a+1/b)

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222-9+11+12:2*14+14 = ? ( )

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