Yajat Special (Problem 1 1 )

Algebra Level 2

log 2 x 6 log x 2 = 5 \log_2x - 6 \log_x 2 = 5

Can the equation above be solved?

No Yes

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2 solutions

Tom Engelsman
Nov 13, 2020

Let u = log 2 x u = \log_{2}x so that we obtain the quadratic equation:

u 6 u = 5 u 2 5 u 6 = ( u 6 ) ( u + 1 ) = 0 u = 1 , 6. u - \frac{6}{u} = 5 \Rightarrow u^2 - 5u - 6 = (u-6)(u+1) = 0 \Rightarrow u = -1, 6.

which gives back log 2 x = 1 , 6 x = 1 2 , 64 \log_{2}x = -1, 6 \Rightarrow x =\frac{1}{2}, 64 . Since x R + x \in \mathbb{R^{+}} , the answer is YES.

Chew-Seong Cheong
Nov 13, 2020

Given that

log 2 x 6 log x 2 = 5 log 2 x 6 log 2 2 log 2 x = 5 log 2 x 6 1 log 2 x = 5 log 2 2 x 5 log 2 x 6 = 0 ( log 2 x 6 ) ( log 2 x + 1 ) = 0 \begin{aligned} \log_2 x - 6 \log_x 2 & = 5 \\ \log_2 x - 6 \cdot \frac {\log_2 2}{\log_2 x} & = 5 \\ \log_2 x - 6 \cdot \frac 1{\log_2 x} & = 5 \\ \log_2^2 x - 5 \log_2 x - 6 & = 0 \\ (\log_2 x - 6)(\log_2 x + 1) & = 0 \end{aligned}

log 2 x = { 6 x = 2 6 = 64 1 x = 2 1 = 1 2 \implies \log_2 x = \begin{cases} 6 & \implies x = 2^6 = 64 \\ -1 & \implies x = 2^{-1} = \dfrac 12 \end{cases}

Yes , it can be solved.

The name of the question is because it came from the deep recesses of my brain (quote taken from Dr. Frost)

Yajat Shamji - 7 months ago

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Hi Yajat, what is happening with problems II and III in this series? I've seen from notifications that you've replied to reports, but it's not possible for users to view those replies and still be able to answer the questions. It doesn't look like the problems have been edited.

Chris Lewis - 6 months, 3 weeks ago

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I am in school. Wait until 3 : 30 3:30 pm.

Yajat Shamji - 6 months, 3 weeks ago

@Chris Lewis

Yajat Shamji - 6 months, 3 weeks ago

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