The ratio of the 11th term of an AP to the 8th term is 23 find the ratio of the 5th term to 21st isn't the ratio of their sums 5:9.
Let the first term and common difference of an A.P. is a and d respectively.
We know that,
Now, we are asked to find . So,
Substitute value of a from equation (3), we get,
Sum of first 5 term and first 21 terms of an A.P. be as,
Substitute value of a from equation (3), we get,
Hence, the ratio of fifth and 21st term is -3 : 1 and the ratio of sum of first 5 terms and first 21 terms is 5 : 9