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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 Bryant Williams. in Ghana
Feb 15,2025

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Answer

p = 6 and q = 4

Solution

We are given that the first three terms of a geometric series are   u₁ = q(4p + 1)   u₂ = q(2p + 3)   u₃ = q(2p – 3) and that the sum to infinity of the series is 250. Step 1. Find p. For a geometric series the ratio of successive terms is constant. Thus, u₂/u₁ = u₃/u₂. Using the given terms, we have   [q(2p + 3)]² = [q(4p + 1)][q(2p – 3)]. Since q ≠ 0 we can cancel q²:   (2p + 3)² = (4p + 1)(2p – 3). Expanding both sides:   LHS: (2p + 3)² = 4p² + 12p + 9.   RHS: (4p + 1)(2p – 3) = 8p² – 10p – 3. Setting them equal gives:   4p² + 12p + 9 = 8p² – 10p – 3. Rearrange by subtracting the left side from the right side:   0 = 8p² – 10p – 3 – 4p² – 12p – 9   0 = 4p² – 22p – 12. Divide the entire equation by 2 to simplify:   0 = 2p² – 11p – 6. This quadratic factors or can be solved using the quadratic formula:   p = [11 ± √(11² – 4·2·(–6))] / (2·2)     = [11 ± √(121 + 48)] / 4     = [11 ± √169] / 4     = [11 ± 13] / 4. Thus, the two possible solutions for p are:   p = (11 + 13)/4 = 24/4 = 6  and  p = (11 – 13)/4 = –2/4 = –½. Since the problem asks for p as an integer, we choose p = 6. Step 2. Find q. Substitute p = 6 into the first three terms to verify the common ratio and then use the sum to infinity formula. Calculate the first three terms:   u₁ = q(4·6 + 1) = q(24 + 1) = 25q,   u₂ = q(2·6 + 3) = q(12 + 3) = 15q,   u₃ = q(2·6 – 3) = q(12 – 3) = 9q. The common ratio is:   r = u₂/u₁ = (15q)/(25q) = 3/5    (and u₃/u₂ = 9q/15q = 3/5). The sum to infinity for a geometric series is given by:   S∞ = u₁ / (1 – r). We are told S∞ = 250, so:   (25q) / (1 – 3/5) = 250. Since 1 – 3/5 = 2/5, the equation becomes:   25q / (2/5) = 250   ⟹  25q × (5/2) = 250   ⟹  (125/2)q = 250. Solve for q by multiplying both sides by 2/125:   q = 250 × (2/125) = 500/125 = 4. Final Answers: a) p = 6 b) q = 4.

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To find the value of \( p \), we start by recognizing the property of a geometric series where the ratio \( r \) between consecutive terms is constant. Thus, we can express this relationship as follows: \[ \frac{u_2}{u_1} = \frac{u_3}{u_2} \] Substituting the values of \( u_1, u_2, \) and \( u_3 \): \[ \frac{q(2p + 3)}{q(4p + 1)} = \frac{q(2p - 3)}{q(2p + 3)} \] Removing \( q \) gives us: \[ \frac{2p + 3}{4p + 1} = \frac{2p - 3}{2p + 3} \] We cross-multiply to eliminate the fractions: \[ (2p + 3)^2 = (4p + 1)(2p - 3) \] Expanding both sides: \[ 4p^2 + 12p + 9 = 8p^2 - 12p + 2p - 3 \] Combining and simplifying terms results in: \[ 4p^2 - 26p + 12 = 0 \] Using the quadratic formula \( p = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \): \[ p = \frac{26 \pm \sqrt{(-26)^2 - 4 \cdot 4 \cdot 12}}{2 \cdot 4} \] Calculating that we find: \[ p = \frac{26 \pm \sqrt{676 - 192}}{8} = \frac{26 \pm \sqrt{484}}{8} = \frac{26 \pm 22}{8} \] Calculating both roots gives: 1. \( p = \frac{48}{8} = 6 \) 2. \( p = \frac{4}{8} = \frac{1}{2} \) Since we need \( p \) as an integer, we take \( p = 6 \). Now, substituting back to find \( q \): The sum to infinity of a geometric series is given by \( S = \frac{u_1}{1 - r} \), and we know \( S = 250 \). First, we find \( r \): \[ r = \frac{u_2}{u_1} = \frac{q(2(6) + 3)}{q(4(6) + 1)} = \frac{q(12 + 3)}{q(24 + 1)} = \frac{15}{25} = \frac{3}{5} \] Substituting \( u_1 \) into the formula for \( S \): \[ 250 = \frac{q(4(6) + 1)}{1 - \frac{3}{5}} = \frac{q(24 + 1)}{\frac{2}{5}} = \frac{25q}{\frac{2}{5}} = \frac{125q}{2} \] Solving for \( q \): \[ 250 = \frac{125q}{2} \] \[ 500 = 125q \implies q = 4 \] Thus, the final values are: a) \( p = 6 \) and b) \( q = 4 \).

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