Question

Find the sum of first 20 terms of the A.P. 1, 4, 7, 10, ..... .​

Answer 1

Answer:

S20=590

Step-by-step explanation:

A.P. 1, 4, 7, 10, .....

a=1, d=4-1=3, n=20

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

S20=20/2[2(1)+(20-1)3]

S20=10(2+(19)3]

S20=10(2+57)

S20=10(59)

S20=590

Therefore, the sum of first 20 terms is 590.

Answer 2

Answer:

590

Step-by-step explanation:

By arthemetic progression formulas,

Sum of n terms = n/2 (2a + (n-1) d)

a, first term = 1

d, common difference = 3

n = 20

Therefore, Sn = 20/2 (2(1) + (20-1) 3)

= 10 (2 + 19*3)

= 10 (2 + 57)

= 10 (59)

= 590