Question

The sum of 20 terms of the series 1 + 4 + 7 + 10 +….is:​

Answer 1

Answer:

Sum of 20 terms of the series is 590.

Step-by-step explanation:

a = 1

d = 4 - 1 = 3

Sₙ = $\frac{n}{2}(2a + (n-1)d)$

S₂₀ = $\frac{20}{2}(2(1)+19(3))$

     = $10(2+57)$

     = $10(59)$

     = $590$

Answer 2

GIVEN :–

• A series => 1 + 4 + 7 + 10 + ..............

TO FIND :–

• Sum of first 20 terms = ?

SOLUTION :–

• Sum of n terms of A.P. is –

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$\bf \large \implies{ \boxed{ \bf S_n = \dfrac{n}{2} \left[ 2a + (n-1)d \right]}}$

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• Here –

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$\bf \: \: \: {\huge{.}} \: \: \: a = 1$

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$\bf \: \: \: {\huge{.}} \: \: \: d = 4 - 1 = 3$

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$\bf \: \: \: {\huge{.}} \: \: \: n = 20$

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• So that –

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$\bf \implies S_{20} = \dfrac{20}{2} \left[ 2(1)+ (20-1)(3) \right]$

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$\bf \implies S_{20} = \cancel \dfrac{20}{2} \left[ 2+ (19)(3) \right]$

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$\bf \implies S_{20} = 10\left[ 2+ (19)(3) \right]$

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$\bf \implies S_{20} = 10(2+ 57)$

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$\bf \implies S_{20} = 10(59)$

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$\bf \implies \large{ \boxed{ \bf S_{20} = 590}}$

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• Hence , Sum of 20 terms is 590.