Question

radius of 11th term of A,P to it's 18th term is 2:3 .find radius of sum of 5.term to sum of term​

Answer 1

Let R= a+(n-1)d= a+(11-1)d=a+10d and

r= a+(n-1)d= a+(18-1)d=a+17d

As per question R/r= 2:3

(a+10d) = 2

(a+17d) 3

3(a+10d)=2(a+17d)

3a+30d=2a+34d

3a-2a= 34d-30d

a= 4d

S5= 5/2[2a+(5-1)d]

putting the value of a in equation

= 5/2[2×4d+4d]

= 5×12d/2

= 30d