Sir please give me a detailed solution of this question in which we have been asked to find the sum of first 10 terms of an AP, if the 12th term of an AP is -13 and the sum of its first four terms is 24.
Book RS Aggarwal, class 10, chapter 5C, question no 26.
T12=−13
S4=24
S10=?
a+11d=−13
⇒a=−13−11d
4/2[2a+3d]=24
⇒2a+3d=12 ………..(1)
Substituting the value of a in equation (1)
−26−22d+3d=12
⇒−19d=38
⇒d=−2
a=−13+22=9
S10=10/2[2(9)+9(−2)]
=0.