Question

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

Answer 1

Step-by-step explanation:

sn = n/2 {2a + ( n-1 )d}

= 20 / 2 { 2 ×1 + ( 20 - 1 ) 3}

= 10 {2+ 57 }

= 10 × 59

= 590

Sn = 590

Answer 2

a= t1 = 1 and d = 3

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

= 20 /2 ( 2(1)+ (20-1) 3)

= 10(2+(19) 3)

= 10(2+57)

= 10×59

Sn = 590