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The second term of a geometric sequence is 2 and the last term is 8. If the common ratio is 2√, how many geometric means are there between the first term and the last term?

 Aug 12, 2015

Best Answer 

 #2
avatar+26368 
+5

The second term of a geometric sequence is 2 and the last term is 8. If the common ratio is 2√, how many geometric means are there between the first term and the last term?

 

$$\small{\text{$
\begin{array}{rcl}
t_2 &=& t_1\cdot r^1 \qquad | \qquad t_2 = 2 \quad \text{ and }\quad r = 2^{\frac 12} \\
2 &=& t_1 \cdot 2^{\frac 12}\\
t_1 &=& 2\cdot 2^{-\frac 12}\\
t_1 &=& 2^{ 1- \frac 12 }\\
\mathbf{t_1} & \mathbf{=} & \mathbf{2^{ \frac 12 }}\\\\\\
\end{array}
$}}$$

$$\small{\text{$
\begin{array}{rcl}
t_n &=& t_1\cdot r^{n-1} \qquad | \qquad t_n = 2^3 \quad \text{ and }\quad t_1 = 2^{ \frac 12 } \quad \text{ and } \quad r = 2^{\frac 12}\\
2^3 &=& 2^{ \frac 12 } \cdot 2^{\frac 12 \cdot(n-1)} \qquad | \qquad 1^2\\
2^6 &=& 2^1 \cdot 2^{n-1}\\
2^6 &=& 2^{1+n-1}\\
2^6 &=& 2^n\\
6 &=& n\\
\mathbf{ n } & \mathbf{=} & \mathbf{6} \\
\end{array}
$}}$$

 

We have 6 terms, so between the first term and the 6th term are 4 terms.

 

$$\small{\text{$
\begin{array}{rcl}
t_1 &=& 2^{\frac 12} = \sqrt{2}\\
t_2 &=& 2^{\frac 22} = 2 \\
t_3 &=& 2^{\frac 32} = 2^{\frac 22}\cdot 2^{\frac 12} = 2\cdot \sqrt{2}\\
t_4 &=& 2^{\frac 42} = 2^2 = 4\\
t_5 &=& 2^{\frac 52} = 2^{\frac 42}\cdot 2^{\frac 12} = 2^2\cdot \sqrt{2}= 4\cdot \sqrt{2}\\
t_6 &=& 2^{\frac 62} = 2^3 = 8\\
\end{array}
$}}$$

 

 Aug 12, 2015
 #1
avatar+118608 
+5

The second term of a geometric sequence is 2 and the last term is 8. If the common ratio is 2√, how many geometric means are there between the first term and the last term?

 

This is the sequence,

 

$$2,\;\;2\sqrt2,\;\;4,\;\;4\sqrt2,\;\;8$$

 

I am not sure what geometric mean means in relation to this question.  :/

 Aug 12, 2015
 #2
avatar+26368 
+5
Best Answer

The second term of a geometric sequence is 2 and the last term is 8. If the common ratio is 2√, how many geometric means are there between the first term and the last term?

 

$$\small{\text{$
\begin{array}{rcl}
t_2 &=& t_1\cdot r^1 \qquad | \qquad t_2 = 2 \quad \text{ and }\quad r = 2^{\frac 12} \\
2 &=& t_1 \cdot 2^{\frac 12}\\
t_1 &=& 2\cdot 2^{-\frac 12}\\
t_1 &=& 2^{ 1- \frac 12 }\\
\mathbf{t_1} & \mathbf{=} & \mathbf{2^{ \frac 12 }}\\\\\\
\end{array}
$}}$$

$$\small{\text{$
\begin{array}{rcl}
t_n &=& t_1\cdot r^{n-1} \qquad | \qquad t_n = 2^3 \quad \text{ and }\quad t_1 = 2^{ \frac 12 } \quad \text{ and } \quad r = 2^{\frac 12}\\
2^3 &=& 2^{ \frac 12 } \cdot 2^{\frac 12 \cdot(n-1)} \qquad | \qquad 1^2\\
2^6 &=& 2^1 \cdot 2^{n-1}\\
2^6 &=& 2^{1+n-1}\\
2^6 &=& 2^n\\
6 &=& n\\
\mathbf{ n } & \mathbf{=} & \mathbf{6} \\
\end{array}
$}}$$

 

We have 6 terms, so between the first term and the 6th term are 4 terms.

 

$$\small{\text{$
\begin{array}{rcl}
t_1 &=& 2^{\frac 12} = \sqrt{2}\\
t_2 &=& 2^{\frac 22} = 2 \\
t_3 &=& 2^{\frac 32} = 2^{\frac 22}\cdot 2^{\frac 12} = 2\cdot \sqrt{2}\\
t_4 &=& 2^{\frac 42} = 2^2 = 4\\
t_5 &=& 2^{\frac 52} = 2^{\frac 42}\cdot 2^{\frac 12} = 2^2\cdot \sqrt{2}= 4\cdot \sqrt{2}\\
t_6 &=& 2^{\frac 62} = 2^3 = 8\\
\end{array}
$}}$$

 

heureka Aug 12, 2015

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