The sum of first q terms of an A.P. is 162. The ratio of its 6th term to its 13th term is 1.: 2. Find the first and 15th term of the A.P
Open in App
Solution
Here is the solution of the problem if the sum of 9 terms of A.P is 162. Given, Sum of terms= 162 Ratio of 6th term and 13th term= 1:2 Formula is an= a1+(n-1)d Then the sixth term a6= a1+5d The 13th term is a13=a1+12d As given a6/a13 is 1/2 (a1+5d)/(a1+12d)=1/2 After cross multiplication 2(a1+5d)=1(a1+12d) 2a1+10d=a1+12d 2a1-a1=12d-10d Therefore the first term a1=2d Sn=(n/2)[2a+(n-1)d] S9=(9/2)[2(2d)+(9-1)d] S9=(9/2)(4d+8d) 162=(9/2)(12d) 162=9(6d) 54d=162 d=3 a1=2d=2(3)=6 a15=6+(15-1)3=6+3(14)=48 The first term of A.P is 6 The 15th term of A.P is 48