Solve the equation ex−2−14(1e)x−2=5e^{x-2} - 14\left(\frac{1}{e}\right)^{x-2} = 5. Solve the equation log2(3x+1)−2log⁡22=log⁡2(2−x2)log_{2}(3x+1) - \dfrac{2}{\log_{\sqrt{2}}2} = \log_{2}(2 - x^{2}).

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Worked solution

4((a))

1. Introduce a positive substitution to simplify the exponential terms.

Let y=ex−2y=e^{x-2}. Since ex−2>0e^{x-2}>0, y>0y>0.

Also,
(1e)x−2=e−(x−2)=1ex−2=1y.{\left(\frac{1}{e}\right)}^{x-2}=e^{-(x-2)}=\frac{1}{e^{x-2}}=\frac{1}{y}.

2. Form and solve the resulting quadratic equation.

The equation becomes
y−14(1y)=5.y-14\left(\frac{1}{y}\right)=5.

Multiply throughout by yy:
y2−14=5yy^2-14=5y
y2−5y−14=0.y^2-5y-14=0.

Factorise:
(y−7)(y+2)=0.(y-7)(y+2)=0.

So y=7y=7 or y=−2y=-2.

3. Reject the invalid substituted value and solve for $x$.

Since y=ex−2>0y=e^{x-2}>0, reject y=−2y=-2.

Thus
ex−2=7.e^{x-2}=7.

Taking natural logarithms,
x−2=ln⁡7x-2=\ln 7
x=2+ln⁡7.x=2+\ln 7.

Answer

x=2+ln⁡7x=2+\ln 7

4((b))

1. Simplify the constant involving the logarithm with base $\sqrt{2}$.

Since 2=212\sqrt{2}=2^{\frac12},
log⁡22=log⁡2122=2.\log_{\sqrt{2}}2=\log_{2^{\frac12}}2=2.
Hence
2log⁡22=22=1.\frac{2}{\log_{\sqrt{2}}2}=\frac{2}{2}=1.

2. Combine the logarithmic terms into one logarithm equation.

The equation becomes
log⁡2(3x+1)−1=log⁡2(2−x2).\log_2(3x+1)-1=\log_2(2-x^2).
Since 1=log⁡221=\log_2 2,
log⁡2(3x+1)−log⁡22=log⁡2(2−x2),\log_2(3x+1)-\log_2 2=\log_2(2-x^2),
log⁡2(3x+12)=log⁡2(2−x2).\log_2\left(\frac{3x+1}{2}\right)=\log_2(2-x^2).

3. Equate the arguments and solve the resulting quadratic equation.

Since the logarithms have the same base,
3x+12=2−x2.\frac{3x+1}{2}=2-x^2.
3x+1=4−2x23x+1=4-2x^2
2x2+3x−3=0.2x^2+3x-3=0.
Using the quadratic formula,
x=−3±32−4(2)(−3)2(2)x=\frac{-3\pm\sqrt{3^2-4(2)(-3)}}{2(2)}
x=−3±334.x=\frac{-3\pm\sqrt{33}}{4}.

4. Check the domain restrictions of the logarithms and reject invalid solution.

For the logarithms to be defined,
3x+1>0and2−x2>0.3x+1>0 \quad \text{and} \quad 2-x^2>0.
So
x>−13and−2<x<2.x>-\frac13 \quad \text{and} \quad -\sqrt2<x<\sqrt2.
Now
−3+334≈0.686,\frac{-3+\sqrt{33}}{4}\approx0.686,
which is valid, but
−3−334≈−2.186,\frac{-3-\sqrt{33}}{4}\approx-2.186,
which is invalid.
Therefore,
x=−3+334.x=\frac{-3+\sqrt{33}}{4}.

Answer

x=−3+334x=\dfrac{-3+\sqrt{33}}{4}

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