Question

The sum of first 15 term 3,7,11......

Answer 1

Step-by-step explanation:

3,7,11... forms an A.P.

With common difference = 7-3=4

Sum of n terms of A.P.= n/2[2a+(n-1)d]

we have :

a= 3

d=4

n=15

substituting values in formula we get

15/2[2×3+(15-1)4]

by solving this we got

SUM = 420

Answer 2

Answer:

Step-by-step explanation:

3, 7, 11, ... is a Arithmetic progression.

which is a or First Number  = 3

The Progression ( b )  = U2 - U1  

                                     = 7 - 3

                                     = 4

sum of Arimethic Progression is :

Sn = n/2 ( 2a + ( n - 1 ) b ) ==> n define how many term the question asked

S15 = 15/2 ( 2(3) + ( 15 - 1 ) 4)

      = 7.5 ( 6 + (14) 4 )

      = 7.5 ( 6 + 56 )

      = 7.5 ( 62 )

      = 465

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