The problem asks us to solve the exponential equation $4 \times 2^{2x+1} = 80$ for $x$.

AlgebraExponential EquationsLogarithmsEquation Solving
2025/4/23

1. Problem Description

The problem asks us to solve the exponential equation 4×22x+1=804 \times 2^{2x+1} = 80 for xx.

2. Solution Steps

First, divide both sides of the equation by 4:
4×22x+1=804 \times 2^{2x+1} = 80
22x+1=8042^{2x+1} = \frac{80}{4}
22x+1=202^{2x+1} = 20
Next, we can take the logarithm of both sides. Let's use the base-2 logarithm.
log2(22x+1)=log2(20)log_2(2^{2x+1}) = log_2(20)
Using the logarithm property logb(ac)=clogb(a)log_b(a^c) = c \cdot log_b(a), we get:
(2x+1)log2(2)=log2(20)(2x+1)log_2(2) = log_2(20)
Since log2(2)=1log_2(2) = 1, we have:
2x+1=log2(20)2x+1 = log_2(20)
Subtract 1 from both sides:
2x=log2(20)12x = log_2(20) - 1
Divide both sides by 2:
x=log2(20)12x = \frac{log_2(20) - 1}{2}
We can express log2(20)log_2(20) as log2(4×5)=log2(4)+log2(5)=log2(22)+log2(5)=2+log2(5)log_2(4 \times 5) = log_2(4) + log_2(5) = log_2(2^2) + log_2(5) = 2 + log_2(5).
Therefore,
x=2+log2(5)12=1+log2(5)2x = \frac{2 + log_2(5) - 1}{2} = \frac{1 + log_2(5)}{2}
We can rewrite this as:
x=12+log2(5)2=12+12log2(5)=12+log2(51/2)=12+log2(5)x = \frac{1}{2} + \frac{log_2(5)}{2} = \frac{1}{2} + \frac{1}{2}log_2(5) = \frac{1}{2} + log_2(5^{1/2}) = \frac{1}{2} + log_2(\sqrt{5}).
Alternatively, we can find an approximate value using a calculator.
x=log2(20)12x = \frac{log_2(20) - 1}{2}. We know that 24=162^4=16 and 25=322^5=32, so log2(20)log_2(20) is between 4 and

5. Using a calculator, $log_2(20) \approx 4.3219$.

x4.321912=3.321921.66095x \approx \frac{4.3219 - 1}{2} = \frac{3.3219}{2} \approx 1.66095

3. Final Answer

x=log2(20)12=1+log2(5)2=12+log2(5)x = \frac{log_2(20) - 1}{2} = \frac{1 + log_2(5)}{2} = \frac{1}{2} + log_2(\sqrt{5})

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