Solution 3.4:2c

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m (Lösning 3.4:2c moved to Solution 3.4:2c: Robot: moved page)
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Regardless of what value
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<center> [[Image:3_4_2c.gif]] </center>
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<math>x</math>
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{{NAVCONTENT_STOP}}
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has, both sides of the equation are positive, because they are of the type "positive number raised to something". It is therefore possible to take the logarithm of both sides and, by using the log laws, to get
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<math>x</math>
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down from the exponents:
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LHS
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<math>=\ln \left( 3e^{x^{2}} \right)=\ln 3+\ln e^{x^{2}}=\ln 3+x^{2}\ln e=\ln 3+x^{2}</math>
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RHS
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<math>=\ln 2^{x}=x\ln 2</math>
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If we collect the terms onto one side, the equation becomes
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<math>x^{2}-x\centerdot \ln 2+\ln 3=0</math>
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which is a standard second-order equation for which we complete the square:
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<math>\begin{align}
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& \left( x-\frac{1}{2}\ln 2 \right)^{2}-\left( \frac{1}{2}\ln 2 \right)^{2}+\ln 3=0 \\
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& \left( x-\frac{1}{2}\ln 2 \right)^{2}=\left( \frac{1}{2}\ln 2 \right)^{2}-\ln 3 \\
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\end{align}</math>
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Now, we have to be cautious and remember that, because
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<math>\text{2}<e<\text{3}</math>
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and thus
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<math>\text{ln 2}<\text{1}<\text{ln 3}</math>
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which means that
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<math>\frac{1}{4}\left( \ln 2 \right)^{2}<\ln 3</math>
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and the right-hand side is therefore negative. Since the square on the right-hand side cannot be negative, the equation has no solution.

Revision as of 11:28, 26 September 2008

Regardless of what value \displaystyle x has, both sides of the equation are positive, because they are of the type "positive number raised to something". It is therefore possible to take the logarithm of both sides and, by using the log laws, to get \displaystyle x down from the exponents:

LHS \displaystyle =\ln \left( 3e^{x^{2}} \right)=\ln 3+\ln e^{x^{2}}=\ln 3+x^{2}\ln e=\ln 3+x^{2}

RHS \displaystyle =\ln 2^{x}=x\ln 2


If we collect the terms onto one side, the equation becomes


\displaystyle x^{2}-x\centerdot \ln 2+\ln 3=0


which is a standard second-order equation for which we complete the square:


\displaystyle \begin{align} & \left( x-\frac{1}{2}\ln 2 \right)^{2}-\left( \frac{1}{2}\ln 2 \right)^{2}+\ln 3=0 \\ & \left( x-\frac{1}{2}\ln 2 \right)^{2}=\left( \frac{1}{2}\ln 2 \right)^{2}-\ln 3 \\ \end{align}

Now, we have to be cautious and remember that, because \displaystyle \text{2} and thus \displaystyle \text{ln 2}<\text{1}<\text{ln 3} which means that \displaystyle \frac{1}{4}\left( \ln 2 \right)^{2}<\ln 3 and the right-hand side is therefore negative. Since the square on the right-hand side cannot be negative, the equation has no solution.

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