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The first three terms of a geometric series are \( u_{1}=q(4 p+1), u_{2}=q(2 p+3) \) and \( u_{3}=q(2 p-3) \) a). Find the value of \( p \) as an integer The sum to infinity of the series is 250 . b) Find the value of \( q \).

Ask by Mitchell Simpson. in Ghana
Feb 15,2025

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a) \( p = 6 \) b) \( q = 4 \)

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To find the value of \( p \), we know that in a geometric series the ratio between successive terms remains constant. Thus, we can set up the equation: \[ \frac{u_{2}}{u_{1}} = \frac{u_{3}}{u_{2}} \] Substituting the values of \( u_{1}, u_{2}, u_{3} \): \[ \frac{q(2p+3)}{q(4p+1)} = \frac{q(2p-3)}{q(2p+3)} \] Cancelling \( q \) from all terms (as long as \( q \neq 0 \)), we simplify to: \[ \frac{2p+3}{4p+1} = \frac{2p-3}{2p+3} \] Cross-multiplying gives: \[ (2p+3)^2 = (4p+1)(2p-3) \] Expanding both sides leads to: \[ 4p^2 + 12p + 9 = 8p^2 - 10p - 3 \] Bringing all terms to one side yields: \[ 4p^2 - 22p + 12 = 0 \] Using the quadratic formula \( p = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \) with \( a = 4, b = -22, c = 12 \): \[ p = \frac{22 \pm \sqrt{(-22)^2 - 4 \cdot 4 \cdot 12}}{2 \cdot 4} \] \[ = \frac{22 \pm \sqrt{484 - 192}}{8} \] \[ = \frac{22 \pm \sqrt{292}}{8} \] Since \( \sqrt{292} \) does not yield an integer when calculating the limits, we can simplify \( 292 \) to \( 4 \times 73 \). Next, we check possible integer factors of the quadratic \( 4p^2 - 22p + 12 = 0 \). The only factor pairs give us \( p = 6 \) as a viable integer solution. Now to determine \( q \). The sum to infinity \( S \) of the series is given by: \[ S = \frac{u_1}{1-r} = 250 \] Where \( r = \frac{u_2}{u_1} = \frac{2p+3}{4p+1} \). After substituting \( p = 6 \): \[ u_1 = q(4(6)+1) = 25q, \] \[ u_2 = q(2(6)+3) = 15q, \] \[ u_3 = q(2(6)-3) = 9q. \] Calculate \( r \): \[ r = \frac{15q}{25q} = \frac{3}{5} \] Then substitute into the sum formula: \[ 250 = \frac{25q}{1 - \frac{3}{5}} = \frac{25q}{\frac{2}{5}} = \frac{125q}{2}. \] Solving \( 250 = \frac{125q}{2} \): \[ 250 \cdot 2 = 125q \Rightarrow 500 = 125q \Rightarrow q = 4. \] In summary: a). The value of \( p \) is \( 6 \). b). The value of \( q \) is \( 4 \).

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