My Very Own Twist of a Problem!

Algebra Level 2

Given log 10 12 , b = log 10 6 , c = log 10 2 \log_{10} 12, b = \log_{10} 6, c = \log_{10} \sqrt{2}

What is the value of 9 a b c 9^{a-b-c} ?

82 0 1.39 729

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4 solutions

Dhruva Patil
Jan 22, 2015

a b c = log 10 12 log 10 6 log 10 2 = log 10 2 log 10 2 2 = log 10 2 10 log 10 2 = 2 = 1.44 0 < 9 log 10 2 < 10 log 10 2 T h e o n l y o p t i o n t h a t s a t i s f i e s t h a t i s A : 1.39 a-b-c=\log _{ 10 }{ 12 } -\log _{ 10 }{ 6 } -\log _{ 10 }{ \sqrt { 2 } } =\log _{ 10 }{ 2 } -\frac { \log _{ 10 }{ 2 } }{ 2 } =\log _{ 10 }{ \sqrt { 2 } } \\ { 10 }^{ \log _{ 10 }{ \sqrt { 2 } } }=\sqrt { 2 } =1.44\\ 0<{ 9 }^{ \log _{ 10 }{ \sqrt { 2 } } }<{ 10 }^{ \log _{ 10 }{ \sqrt { 2 } } }\\ The\quad only\quad option\quad that\quad satisfies\quad that\quad is\quad \boxed{A:1.39}

Mehul Chaturvedi
Jan 22, 2015

The given values can be rewritten as

1 0 a = 12 1 0 b = 6 1 0 c = 2 1 0 2 c = 2 10^a=12 \\ 10^b=6 \\ 10^c=\sqrt{2} \Rightarrow 10^{2c}=2

As we know 6 × 2 = 12 6\times 2=12

1 0 b × 1 0 2 c = 1 0 a \therefore 10^b\times 10^{2c}=10^a

1 0 b + 2 c = 1 0 a \large10^{b+2c}=10^a

b + 2 c = a a b = 2 c \therefore b+2c=a \Rightarrow a-b=2c

we have to find 9 a b c 9 2 c c 9 c \large 9^{a-b-c} \Rightarrow 9^{2c-c} \Rightarrow 9^{c}

9 log 2 1 2 3 log 2 1.39 \large 9^{\log{2}^{\frac{1}{2}}} \Rightarrow 3^{\log{2}} \Rightarrow 1.39

Hence our answer is 1.39 \boxed{1.39}

I believe that the problem creator wanted to ask for 9 a b 2 c 9 ^ { a - b - 2c } instead.

Calvin Lin Staff - 6 years, 4 months ago

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I too think that

Mehul Chaturvedi - 6 years, 4 months ago

No...this question was in my School Maths book..I just wanted to share this question :D

Harsh Kumar - 6 years, 3 months ago

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What is the name of that book

Meet Patel - 6 years, 3 months ago

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@Meet Patel S.Chand's ICSE Mathematics for class IX

Harsh Kumar - 6 years, 2 months ago
Jesse Nieminen
Aug 17, 2015

a = log 10 12 , b = log 10 6 , c = log 10 2 {a = \log_{10}12, b = \log_{10}6, c = \log_{10} \sqrt{2}} a b c = log 10 12 log 10 6 log 10 2 = log 10 2 {{a - b - c} = {\log_{10}12} - {\log_{10}6} - {\log_{10} \sqrt{2}} = {\log_{10}\sqrt{2}}} 9 log 10 2 = 3 log 10 2 1.39 {{9}^{\log_{10}\sqrt{2}} = {3}^{\log_{10}2} \approx \boxed {1.39}}

Moderator note:

Simple standard approach.

Michael Stevenson
Jan 22, 2015

With these options, it's easy to find the right one without manipulation. a is a little over one, b somewhat under one, and c can't be much at all. 1.39 is the only answer remotely between 9^0 and 9^1.

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