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Mathematics

The fourth term of a G.P. is eight times its seventh term. The fifth term of then G.P. is 316\dfrac{3}{16}, then find its 12th term.

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Answer

By formula,

an = arn - 1

Given,

The fourth term of a G.P. is eight times its seventh term.

a4 = 8a7

ar4 - 1 = 8ar7 - 1

ar3 = 8ar6

r6r3=a8a\dfrac{r^6}{r^3} = \dfrac{a}{8a}

r3=18r^3 = \dfrac{1}{8}

r = 183=12\sqrt[3]{\dfrac{1}{8}} = \dfrac{1}{2}.

Given,

Fifth term = 316\dfrac{3}{16}

ar4=316a×(12)4=316a×116=316a=3.\Rightarrow ar^4 = \dfrac{3}{16} \\[1em] \Rightarrow a \times \Big(\dfrac{1}{2}\Big)^4 = \dfrac{3}{16} \\[1em] \Rightarrow a \times \dfrac{1}{16} = \dfrac{3}{16} \\[1em] \Rightarrow a = 3.

12th term :

a12 = ar12 - 1

= ar11

= 3×(12)113 \times \Big(\dfrac{1}{2}\Big)^{11}.

Hence, 12th term = 3×(12)113 \times \Big(\dfrac{1}{2}\Big)^{11}.

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