What's the fuss about m m ?

Algebra Level 3

( log 2 m ) 2 + log 2 m 3 = 10 \large ( \log_2 m)^2 + \log_2 m^3 = 10

Find the product of the value(s) of m m that satisfy the equation above.


The answer is 0.125.

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2 solutions

Arjen Vreugdenhil
Mar 28, 2016

Let x = log 2 m x = \log_2 m , then the equation can be written as x 2 + 3 x 10 = 0. x^2 + 3x - 10 = 0. The sum of the zeroes of a quadratic is the negative of the linear coefficient, so x 1 + x 2 = 3. x_1 + x_2 = -3. The product of the corresponding m m values is m 1 m 2 = 2 x 1 + x 2 = 2 3 = 0.125 . m_1 \cdot m_2 = 2^{x_1 + x_2} = 2^{-3} = \boxed{0.125}. Note that it is not even necessary to solve for the individual values of m m or x x ...

Holi smokes, didn't think of that xD

Ahmad Naufal Hakim - 5 years, 2 months ago
Rohit Udaiwal
Mar 25, 2016

( log 2 m ) 2 + log 2 m 3 = 10 ( log 2 m ) 2 + 3 log 2 m 10 = 0 [ log a b c = c log a b ] ( a 2 ) ( a + 5 ) = 0 [ Let log 2 m be a ] a = 2 , 5 log 2 m = 2 , 5 m = 4 , 1 32 \begin{aligned} (\log_{2}{m})^2+\log_{2}{m^3} = & 10 \\ \implies (\log_{2}{m})^2+3\log_{2}{m}-10=& 0 \quad \quad \quad [\log_{a}{b^{c}}=c\log_{a}{b}] \\ \implies (a-2)(a+5)=& 0 \quad \quad \quad [\text{Let}~ \log_{2}{m}~ \text{be}~ a] \\ \implies a=& 2,-5 \implies \log_{2}{m}=2,-5 \\ \therefore m=& 4,\dfrac{1}{32} \end{aligned}

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