Question

Q4. Find the sum of first 20 terms of the following AP:
1, 4, 7, 10....​

Answer 1

Answer:

so using the arithmetic progression formulae :

n/2 {2a + (n + 1)d}

HERE,

n = 20

a = 1

d = 4-1 or 7-4 or 10-7 etc. (a2-a1 basically) = 3

putting values-

20/2 {2*1 + (20+1) x 3}

10{2+21*3}

10 (2 + 63)

10*65

= 650

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cheers : )

Answer 2

Answer:

1,4,7,10.......

a = 1

d = 4-1 = 3

Sn = n [2a + (n- 1 ) d]

2

S20 = 20 [ 2×1 + ( 20 - 1) ×3]

2

= 10 [ 2 + ( 19 ) × 3]

= 10 [ 2 + 57 ]

= 10 × 59

= 590 ANS.