Question

let An be anA.P , it a1 =7 and d = 4, determine sum of the first 6 elements with indexes

Answer 1

Answer:

I don't understand please give minutes

Answer 2

Answer:

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