Q. 374.9( 9 Votes )

If the 12th term of an A.P. is - 13 and the sum of the first four terms is 24, what is the sum of the first 10 terms?

Answer :

Given: a12 = -13


a + 11d = -13


a = -13 – 11d …(i)


and S4 = 24


[from(i)]


2[-26 -22d + 3d] = 24


-26 – 19d = 12


-19d = 12 + 26


-19d = 38


d = -2


Putting the value of d in eq. (i), we get


a = -13 – 11(-2) = -13 + 22 = 9


So, a = 9 , d = -2 and n = 10


Now, we have to find the S10



S10 = 5[2×9 + 9(-2)]


S10 = 5[18 – 18]


S10 = 0


Hence, the sum of first 10 terms is 0


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