let An be anA.P , it a1 =7 and d = 4, determine sum of the first 6 elements with indexes
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I don't understand please give minutes
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Step-by-step explanation:
Sum of even indexes means
the sum of a2 , a4 , a6....
So sum of first 6 even indexes is
a2 + a4 + a6 + a8 + a10 + a12
(a + d) + ( a + 3d) + (a + 5d) + ( a + 7d) + ( a + 9d ) + ( a + 11d)
= 6a + d( 1 + 3 + 5 + 7 + 9 + 11 )
= 6a + 36d
= 6 * 7 + 36 * 4
= 42 + 144
= 186
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