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Algebra Level 5

How many statements are always true?

  • log 2 ( x 45 ) = 45 log 2 ( x ) \log_{2} (x^{45}) = 45\log_{2} (x)

  • log 2 x y = log 2 x + log 2 y \log_{2} xy = \log_{2} x + \log_{2} y

  • log 3 x = 1 log x 3 \log_{3}x = \dfrac{1}{\log_{x}3}

  • log 3 8 = log 156 8 log 156 3 \log_{3}8 = \dfrac{\log_{156}8}{\log_{156}3}


x x is a real number not equal to 0 0 .

log 10 100 \log_{10}100 log 10 1000 \log_{10}1000 log 10 10 \log_{10}10 log 10 10000 \log_{10}10000 log 10 1 \log_{10}1

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1 solution

Harsh Shrivastava
May 13, 2016

Let

A : log 2 ( x 45 ) = 45 log 2 ( x ) A : \log_{2} (x^{45}) = 45\log_{2} (x)

B : log 2 x y = log 2 x + log 2 y B : \log_{2} xy = \log_{2} x + \log_{2} y

C : log 3 x = 1 log x 3 C : \log_{3}x = \dfrac{1}{\log_{x}3}

D : log 3 8 = log 156 8 log 156 3 D : \log_{3}8 = \dfrac{\log_{156}8}{\log_{156}3}

A A is true.

B B is true iff x > 0 x>0 .Thus correct statement should be log 2 x y = log 2 x + log 2 y \log_{2} xy = \log_{2}\color{#3D99F6}{ |x|} + \log_{2}\color{#3D99F6}{| y|} .

C C is true only if x 1 \color{#3D99F6}{x\neq1} .

D D is true. See here .

Domain of LHS and RHS in A is x > 0 x>0 so isnt A true?

Aman Dubey - 5 years, 1 month ago

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Oh sorry, I made a silly mistake.

@Calvin Lin

Harsh Shrivastava - 5 years, 1 month ago

Statement 1 is also true, check it again ...

RAJ RAJPUT - 5 years, 1 month ago

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Oh sorry, I made a silly mistake.

@Calvin Lin

Harsh Shrivastava - 5 years, 1 month ago

@Harsh Shrivastava ,I think A is true! actually x can not be negative neither in LHS nor in RHS as it is not in domain. In case you had written power any number even instead of 45(which is odd), you were right at your claim.

Mayank Chaturvedi - 5 years, 1 month ago

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Oh sorry, I made a silly mistake.

@Calvin Lin

Harsh Shrivastava - 5 years, 1 month ago

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There is no need to mention me in every single comment that you made. Once is enough.

Calvin Lin Staff - 5 years, 1 month ago

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@Calvin Lin I simply copy/pasted my comments.

Harsh Shrivastava - 5 years, 1 month ago

Statement 1 is true. Harsh, that only apply when the exponent is even. If you have an odd exponent (as here), it will be negative in both sides.

No Kia - 5 years, 1 month ago

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Oh sorry, I made a silly mistake.

@Calvin Lin

Harsh Shrivastava - 5 years, 1 month ago

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