Question

In an A.p. if the 12th term is -13 and the sum of first four term is 24.what is the sum of the first 10 term (use formula).

Answer 1

not sure about this answer...

Answer 2

given is 12 term is equal to -13
let the first term be a
common difference be d
and sum of first terms be S4
total number of terms be n
now,
a + 11d = -13. ..............(1 )

again,
S4 = 24
n/2{2a + (n-1)d} = 24
4/2{2a + (4-1)d} = 24
2 {2a + 3d} = 24
or 2a + 3d = 12. ............(2)

solving by elimination method..
eq (1) mlutiplied by 2

2a + 3d = 12
2a + 22d= -26
- - +
19 d = 38
d= 2

by 5 putting the value of d in any of the equations we get a = -35

again,
sum of 10 terms = S4
n/2 {2a + (n-1)d}
= 10/2 {-70+18}
= -260



hope it helps you..