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Quadratic Equation and Inequalities question

2012 · Shift 1 · Q40
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  5. /2012 · Shift 1 · Q40

Quadratic Equation and Inequalities question

2012 · Shift 1 · Q40

JEE AdvancedMathematicsQuadratic Equation and InequalitiesNumerical+4 / −1
The value of 6+log⁡3/2(1324−1324−1324−132...)6 + {\log _{3/2}}\left( {{1 \over {3\sqrt 2 }}\sqrt {4 - {1 \over {3\sqrt 2 }}\sqrt {4 - {1 \over {3\sqrt 2 }}\sqrt {4 - {1 \over {3\sqrt 2 }}...} } } } \right)6+log3/2​​32​1​4−32​1​4−32​1​4−32​1​...​​​​ is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 4

  1. Let the infinite nested expression be

x=1324−1324−1324−⋯x=\frac{1}{3\sqrt2}\sqrt{4-\frac{1}{3\sqrt2}\sqrt{4-\frac{1}{3\sqrt2}\sqrt{4-\cdots}}}x=32​1​4−32​1​4−32​1​4−⋯​​​

Then the given quantity is

6+log⁡3/2(x).6+\log_{3/2}(x).6+log3/2​(x).


  1. Use self-similarity of the infinite radical

Since the same expression repeats inside the square root, we can write

x=1324−x.x=\frac{1}{3\sqrt2}\sqrt{4-x}.x=32​1​4−x​.

Now solve for xxx.


  1. Square both sides

x2=118(4−x)x^2=\frac{1}{18}(4-x)x2=181​(4−x)

because

(132)2=118.\left(\frac{1}{3\sqrt2}\right)^2=\frac{1}{18}.(32​1​)2=181​.

So,

18x2=4−x18x^2=4-x18x2=4−x

18x2+x−4=0.18x^2+x-4=0.18x2+x−4=0.


  1. Solve the quadratic equation

18x2+x−4=018x^2+x-4=018x2+x−4=0

Using the quadratic formula,

x=−1±1+28836=−1±1736.x=\frac{-1\pm\sqrt{1+288}}{36}=\frac{-1\pm17}{36}.x=36−1±1+288​​=36−1±17​.

Thus,

x=1636=49orx=−1836=−12.x=\frac{16}{36}=\frac49 \quad \text{or} \quad x=\frac{-18}{36}=-\frac12.x=3616​=94​orx=36−18​=−21​.

Since the nested radical expression is positive, we take

x=49.x=\frac49.x=94​.


  1. Evaluate the logarithm

We need

6+log⁡3/2(49).6+\log_{3/2}\left(\frac49\right).6+log3/2​(94​).

Now,

49=(23)2=(32)−2.\frac49=\left(\frac23\right)^2=\left(\frac32\right)^{-2}.94​=(32​)2=(23​)−2.

Hence,

log⁡3/2(49)=log⁡3/2((32)−2)=−2.\log_{3/2}\left(\frac49\right)=\log_{3/2}\left(\left(\frac32\right)^{-2}\right)=-2.log3/2​(94​)=log3/2​((23​)−2)=−2.

Therefore,

6+log⁡3/2(49)=6−2=4.6+\log_{3/2}\left(\frac49\right)=6-2=4.6+log3/2​(94​)=6−2=4.


  1. Final Answer

4\boxed{4}4​

The derived answer matches the stored correct answer.

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