Question

the first term of an arithmetic sequence is 9 and the sum to 10 terms is 225 .find the sum of first 20 terms​

Answer 1

Step-by-step explanation:

S10 = n /2 [ 9+ ( n - 1 )(d) ]

S10 = 10 /2 [ 9+ (10 -1 ) (d) ]

225 = 5 [ 9 + 9d ]

225 / 5 = 9 + 9d

45 = 9 + 9d

45 - 9 = 9d

36 = 9d

36/ 9 = d

4 = d

S20 = n/ 2 [ a + ( n -1 )(d) ]

S20 = 20 /2 [ 9 + ( 20 -1 ) ( 4) ]

S20 = 10 [ 9 + 76]

S20 = 10 [ 85 ]

S20 = 850