NECO 2004 Mathematics · Question 15
The 4th term of a G.P is 3 and its first term is 81. What is its common ratio?
Options
- A.3
- B.
- C.
- D.-
- E.-3
Answer: C.
Explanation written with AI and checked against the answer key
Analyze the question: We have a geometric progression (G.P.) with first term a = 81 and 4th term T₄ = 3. We need the common ratio r. Concept: The nth term of a G.P. is Tₙ = arⁿ⁻¹ Solution: T₄ = ar³ = 3 81r³ = 3 r³ = 3/81 = 1/27 r = ∛(1/27) = 1/3 Check: 81, 27, 9, 3. The 4th term is 3. ✓ The negative option D (-1/3) would give 81, -27, 9, -3, so the 4th term would be -3, not 3. That makes it wrong. Tip: When you get r³ = some number, take the cube root directly. An odd power keeps the sign, so a positive result means a positive ratio. Also, you can always verify by listing the terms, as we did above.AI can make mistakes. Report anything that looks wrong.
Past questions © the examination body (JAMB/WAEC/NECO). DeeepSolve is not affiliated with them. Rights holders: see /copyright.