How many terms of the AP 25, 28, 31, 34, .... are needed to give the sum 1070?
Answers
Answered by
6
Answer:
Sn = 1070
a = 25
d = 28 - 25 = 3
n = ?
FORMULA : Sn = n/2 ( 2a + (n -1) d)
1070 = n/2 [ 2 x 25 + ( n - 1 ) 3 ]
1070 x 2 = n [ 50 + 3n - 3 ]
2140 = n ( 47 + 3n)
2140 = 47n + 3n²
3n² + 47n - 2140 = 0
3n² + 60n - 107n - 2140 = 0
3n ( n + 20) - 107 ( n + 20) = 0
( 3n - 107) (n + 20) = 0
3n = 107
n = 107 /3
n = 35.6
n = -20
terms required = 36
hope it helps
Similar questions