NumericalHard4 marks
The 4th term of an A.P. is 22 and 15th term is 66. Find the first term and the common difference. Hence find the sum of the series to 8 terms.
Topic: Arithmetic progression
Previous-year question practice database
Answer
Exam answer
T₄ = a+3d = 22. T₁₅ = a+14d = 66.
Subtracting: 11d = 44 d=4.
a+12=22 a=10.
S₈ = 8/2[2a+7d] =4[20+28] =192.
Answer: a=10, d=4, S₈=192.
Explanation
Use the nth-term formula twice, then the A.P. sum formula.