The ratio of the sum of first n terms of two AP's is 7n+1:4n+27. Find the ratio of their 11th terms .
Ans: Let a1, a2... and d1, d2 be the I terms are Cd's of two AP's.
Sn of one AP = 7n+1/4n+27
Sn of II AP
m/2[2a1 +(n-1)d1]/ m/2 [2a2 +(n-1)d2] = 7n+1/4n+27
ð 2a1 +(n-1)d1/ 2a2 +(n-1)d2 = 7n+1/4n +27
ð We have sub n = 21
ð 2a1 +20d1/2a2 +20d2 = 7 x21 + 1/4(21)+27
ð A1 +10d1/a2 +10d2 = 148 /111
ð = 4/3