Common ratio problems

**nightrider456** · March 2nd 2010, 03:50 PM

Been having problems with the solutions to the following problems dealing with finding the common ratio.

Solve for the missing variable and find the common ratio
1.) $<br /> \sqrt{x},3,3\sqrt{x}<br />$

2.) $<br /> m+2,m+4,2m+11<br />$

**Soroban** · March 2nd 2010, 07:05 PM

Hello, nightrider456!

If we are given three terms $a,b,c$ with a common ratio,

. . then: . $\frac{a}{b} \:=\:\frac{b}{c}$

Solve for the variable and find the common ratio.

. . $(1)\;\;\sqrt{x},\;3,\;3\sqrt{x}$

We have: . $\frac{\sqrt{x}}{3} \;=\;\frac{3}{3\sqrt{x}} \quad\Rightarrow\quad 3x \:=\:9 \quad\Rightarrow\quad x \:=\:3$

The three terms are: . $\sqrt{3},\;3,\;3\sqrt{3}$

. . The common ratio is: . $\sqrt{3}$

$(2)\;\;m+2,\;m+4,\;2m+11$

We have: . $\frac{m+2}{m+4} \:=\:\frac{m+4}{2m+11} \quad\Rightarrow\quad 2m^2 + 15m + 22 \:=\:m^2 + 8m + 16$

. . $m^2 + 7m + 6 \:=\:0 \quad\Rightarrow\quad (m+1)(m+6) \:=\:0$

Hence: . $m \;=\;-1,\:-6$ . . . two solutions

If $m = -1$ , the three terms are: . $1,\:3,\:9$
. . The common ratio is: . $3$

If $m = -6$ , the three terms are: . $-4,\:-2,\:-1$
. . The common ratio is: . $\tfrac{1}{2}$

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