Question

Find the sum of last 8 term of a.p -12,-10,-8.........58

Answer 1

$\blue{\bold{\underline{\underline{Answer:}}}}$

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$\green{\underline \bold{Given :}}$

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• A.P = -12 , -10 , -8 , .......... 58

Or

• A.P = -12 , -10 , -8 , ........

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$\red{\underline \bold{To \: Find:}}$

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• Sum of last 8 terms of this AP

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$\underline{\bold{\texttt{We know that :}}}$

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$\purple{\sf S_n = \dfrac { n } { 2 } ( 2a + (n -1)d )}$

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Where,

• n = number od terms
• a = first term
• d = common difference

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$\underline{\bold{\texttt{For common difference }}}$

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⇛ -10 - (-12)

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⇛ -10 + 12

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Common difference = 2

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Let us start from the back side,

• a = 58
• n = 8
• d = -2

[ As d = 2 when starting from front hence by changing the sign we got d = -2 when we start from the back ]

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Putting these values

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$\longmapsto \sf S_n = \dfrac { 10 } { 2 } ( 2(58) + (10 -1)-2 )$

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$\longmapsto \sf S_n = 5( 116 - 18 )$

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$\longmapsto \sf S_n = 5( 98 )$

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$\longmapsto \sf S_n = 490$

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Hence sum of last 8 terms is 490.

$\rule{200}5$

Answer 2

Step-by-step explanation:

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