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If (log)3x=aa n d(log)7x=b , then which...

If `(log)_3x=aa n d(log)_7x=b ,` then which of the following is equal to `(log)_(21)x ?` a.`a b` b. `(a b)/(a^(-1)+b^(-1))` c. `1/(a+b)` d. `1/(a^(-1)+b^(-1)`

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