Question

Find the sum of 20 terms of an Ap 2. 7. 12 using suitable formula

Answer 1

Step-by-step explanation:

d = a2 - a1

d= 7-2

d= 5

a20 = a+ (n-1) d

a20 = 2+(20-1) 5

a20 = 2+19×5

a20 = 2+95

a20 = 97

I hope it helps you

Answer 2

Sum of 20 terms is 990

Step-by-step explanation:

GIVEN

a = t1 = 2

t2 = 7

t3 = 12

___________________________

TO FIND

Sum of 20 terms

___________________________

We know that,

Common Difference ( d ) = t2 - t1

Common Difference ( d ) = 12 - 7

Common Difference ( d ) = 5

Common Difference ( d ) = 5

___________________________

We know that,

Sn = n/2 × [ 2a + ( n - 1 ) × d ]

S20 = 20/2 × [ 2 × 2 + ( 20 - 1 ) × 5 ]

S20 = 20/2 × [ 4 + ( 20 - 1 ) × 5 ]

S20 = 20/2 × [ 4 + 19 × 5 ]

S20 = 20/2 × [ 4 + 95 ]

S20 = 20/2 × 99

S20 = 10 × 99

S20 = 990

S20 = 990

Sum of 20 terms is 990