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The sum of the first 5term …

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The sum of the first 5term and the term of first term of AP is 167. It is the sum the first 10th term of this AP is 235 then find the sum of its first twenty term
  • 1 answers

Sia ? 6 years, 8 months ago

 According to the question, S5 + S7 = 167
{tex}\Rightarrow \frac{5}{2}[2 a+4 d]+\frac{7}{2}{/tex}[ 2a + 6d] = 167
{tex}\Rightarrow{/tex} 5(a + 2d) + 7(a + 3d) = 167
{tex}\Rightarrow{/tex} 12a + 31d = 167 ......(i)
and S10 = 235
{tex}\Rightarrow \quad \frac{10}{2}{/tex}[2a + 9d] = 235
{tex}\Rightarrow{/tex} 2a + 9d = {tex}\frac{235}{5}{/tex} = 47 ........(ii)
Multiplying eq. (ii) by 6 and then subtracting from (i), we have

d = {tex}\frac{-115}{-23}{/tex} = 5
From (ii), we get
2a + 9d = 47
{tex}\Rightarrow{/tex} 2a + 9(5) = 47
{tex}\Rightarrow{/tex} 2a = 47 - 45
{tex}\Rightarrow{/tex} 2a = 2 {tex}\Rightarrow{/tex} a = 1
Hence, S20 = {tex}\frac{20}{2}{/tex}[ 2a + 19d]
= 10[2(1) + 19(5)] = 10[2 + 95] = 970

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