<\/span><\/h2>\n\n\n\nSeries of the form a (b) + (a + d) (br) + (a + 2d) (br2<\/sup>) + (a + 3d) (br3<\/sup>) + ……… + (a + (n – 1) d) (brn-1<\/sup>) + ………<\/em><\/strong> is called Arithmetico-Geometric Series (AG Series) which consists of two series i.e.<\/p>\n\n\n\n- Arithmetic Series \u2192 a + (a + d) + (a + 2d) + ………<\/strong><\/li>
- Geometric Series \u2192 b + br + br2<\/sup> + ………<\/strong><\/li><\/ul>\n\n\n\n
<\/span>Sum to n-terms of Arithmetico-Geometric Series:<\/span><\/h2>\n\n\n\nLet Sn<\/sub> = a + (a + d) r + (a + 2d) r2<\/sup> + ………<\/strong> + (a + (n – 1) d) rn-1<\/sup> ………<\/strong>…. (i) Multiply (i) by r (common ratio):- rSn<\/sub> = ar + (a + d) r2<\/sup> + (a + 2d) r3<\/sup> + ………<\/strong> + (a + (n – 1) d) rn<\/sup> ………<\/strong>…. (ii) Subtract (ii) from (i):- Sn<\/sub> – rSn<\/sub>= a + dr + dr2<\/sup> + dr3<\/sup> + ………<\/strong> + drn-1<\/sup> – (a + d) rn<\/sup> \u21d2 (1 – r) Sn<\/sub> = a + (dr) (rn-1<\/sup> – 1)\/(r – 1) – (a + d) rn<\/sup> \u21d2 Sn<\/sub> = a\/(1 – r) + (dr) (1 – rn-1<\/sup>)\/(1 – r)2<\/sup> – (a + d) rn<\/sup>\/(1 – r)<\/em><\/strong><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<\/span>Sum of Infinite Term of Arithmetico-Geometric Series:<\/span><\/h2>\n\n\n\nLet S\u221e<\/sub> = a + (a + d) r + (a + 2d) r2<\/sup> + …….. \u221e ………<\/strong>…. (i) Multiplying (i) by r:- rS\u221e<\/sub> = ar + (a + d) r2<\/sup> + (a + 2d) r3<\/sup> + …….. \u221e ………<\/strong>…. (ii) Subtract (ii) from (i):- S\u221e<\/sub> – rS\u221e<\/sub>= a + dr + dr2<\/sup> + …….. \u221e \u21d2 (1 – r) S\u221e<\/sub>= a + dr\/(1 – r) \u21d2 S\u221e<\/sub>= a\/(1 – r) + dr\/(1 – r)2<\/sup><\/em><\/strong><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nExample- Show that 1 + 2x + 3x2<\/sup> + …. to n terms (x < 1) is (1 – xn<\/sup>)\/(1 – x)2<\/sup> – nxn<\/sup>\/(1 – x).<\/mark><\/em><\/strong> Find also the sum when n \u2192 \u221e.<\/mark><\/em><\/strong>
Solution-<\/em><\/strong> The given series is, 1 + 2x + 3x2<\/sup> + …… to n terms
The corresponding A.P. is 1, 2, 3, ……… It’s nth<\/sup> term is 1 + (n – 1) 1 = 1 + n – 1 =n
The corresponding G.P. is 1, x, x2<\/sup>, ……… Its nth <\/sup>term is 1. xn-1<\/sup> = xn-1<\/sup>
\u2234 the nth<\/sup> term of the given series = (nth<\/sup> term of A.P.) (nth<\/sup> term of G.P.) = nxn-1<\/sup>
Let Sn<\/sub> = 1 + 2x + 3x2<\/sup> + …… + (n – 1) xn-2<\/sup> + nxn-1<\/sup> ………<\/strong>…. (i) Multiply (i) by x:- xSn<\/sub> = x + 2x2<\/sup> + 3x3<\/sup> + …… + (n – 1) xn-1<\/sup> + nxn<\/sup> ………<\/strong>…. (ii) Subtract (ii) from (i):- Sn<\/sub> – xSn<\/sub> = 1 + x + x2<\/sup> + x3<\/sup> + …… + xn-1<\/sup> – nxn<\/sup> \u21d2 (1 – x) Sn<\/sub> = Sum of n term of G.P. with a = 1, r = x – nxn<\/sup> \u21d2 (1 – x) Sn<\/sub> = [1(1 – xn<\/sup>)\/(1 – x)] – nxn<\/sup> \u21d2 Sn<\/sub> = [(1 – xn<\/sup>)\/(1 – x)2<\/sup>] – [nxn<\/sup>\/(1 – x)] ………<\/strong>…. (A) As n \u2192 \u221e and x < 1 \u2234 xn<\/sup> \u2192 0 \u2234 Equation (A) becomes:- S\u221e<\/sub> = [(1 – 0)\/(1 – x)2<\/sup>] – [n (0)\/(1 – x)] S\u221e<\/sub> = 1\/(1 – x)2<\/sup><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nExample-<\/em><\/strong> Sum to n terms of the series 1 + 2\/3 + 3\/32<\/sup> + 4\/33<\/sup> + ………..<\/mark><\/em><\/strong>
Solution-<\/em><\/strong>The given series is, 1 + 2\/3 + 3\/32<\/sup> + 4\/33<\/sup> + ………..
Let Tn<\/sub> be its nth<\/sup> term. Tn<\/sub> = (nth<\/sup> term of A.P. 1, 2, 3, ………) (nth<\/sup> term of G.P. 1, 1\/3, 1\/32<\/sup>, ………..) \u21d2 Tn<\/sub> = [1 + (n – 1)1] [1 (1\/3)n-1<\/sup>] \u21d2 Tn<\/sub> = n (1\/3)n-1<\/sup>
Let Sn<\/sub> = 1 + 2\/3 + 3\/32<\/sup> + 4\/33<\/sup> + ……….. + (n – 1) 1\/3n-2<\/sup> + n (1\/3n-1<\/sup>) ………<\/strong>…. (i) Multiply (i) by 1\/3 (1\/3) Sn<\/sub> = 1\/3 + 2\/32<\/sup> + 3\/33<\/sup> + ……….. + (n – 1)\/3n-1<\/sup> + n\/3n<\/sup> ………<\/strong>…. (ii)
Subtract (ii) from (i):- Sn<\/sub> – (1\/3) Sn<\/sub> = 1 + 1\/3 + 1\/32<\/sup> + 1\/33<\/sup> + ……….. + 1\/3n-1<\/sup> – n\/3n<\/sup> \u21d2 (1 – 1\/3) Sn<\/sub> = (Sum of n term of G.P. with a = 1, r = 1\/3) – n\/3n<\/sup> \u21d2 (2\/3) Sn<\/sub> = 1[1 – (1\/3n<\/sup>)]\/[1 – (1\/3)] – n\/3n<\/sup> \u21d2 (2\/3) Sn<\/sub> = (3n<\/sup> – 1)\/3n<\/sup>(2\/3) – n\/3n<\/sup> \u21d2 Sn<\/sub> = (3n<\/sup> – 1)\/3n<\/sup>(2\/3)2<\/sup> – n\/(3n<\/sup>) (2\/3) \u21d2 Sn<\/sub> = 1\/(3n<\/sup>) (2\/3) [(3n<\/sup> – 1)\/(2\/3) – n] \u21d2 Sn<\/sub> = 1\/(2) (3n-1<\/sup>) [(3\/2) (3n<\/sup> – 1) – n] \u21d2 Sn<\/sub> = 1\/(2) (3n-1<\/sup>) [(3n+1<\/sup> – 3 – 2n)\/2]<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nExample-<\/em><\/strong> If the sum to infinity of the series 3 + 5r + 7r2<\/sup> + ….. is 44\/9, find r.<\/mark><\/em><\/strong>
Solution-<\/em><\/strong> Let S\u221e<\/sub> = 3 + 5r + 7r2<\/sup> + ….. \u221e ………<\/strong>…. (i) Multiply (i) by r:- rS\u221e<\/sub> = 3r + 5r2<\/sup> + 7r3<\/sup> + ….. ………<\/strong>…. (ii)
Subtract (ii) from (i):- S\u221e<\/sub> – rS\u221e<\/sub> = 3 + 2r + 2r2<\/sup> + 2r3<\/sup> + ….. \u221e \u21d2 (1 – r) S\u221e<\/sub> = 3 + 2r\/(1 – r) \u21d2 S\u221e<\/sub> = 3\/(1 – r) + 2r\/(1 – r)2<\/sup> \u21d2 S\u221e<\/sub> = [3 (1 – r) + 2r]\/(1 – r)2<\/sup> \u21d2 S\u221e<\/sub> = (3 – r)\/(1 – r)2<\/sup>
But S\u221e<\/sub> = 44\/9
\u2234 (3 – r)\/(1 – r)2<\/sup> = 44\/9 \u21d2 27 – 9r = 44 (1 – r)2<\/sup> \u21d2 27 – 9r = 44 (1 + r2<\/sup> – 2r) \u21d2 27 – 9r = 44 + 44r2<\/sup> – 88r \u21d2 -44r2<\/sup> + 88r – 9r = 44 – 27 \u21d2 -44r2<\/sup> + 79r = 17 \u21d2 79r = 44r2<\/sup> + 17 |
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