WAEC 2019 Mathematics · Question 45

The fourth term of an Arithmetic Progression (A.P) is 37 and the first term is -20. Find the common difference.

Options

A.3
B.57
C.19
D.17

Answer: C. 19

Explanation written with AI and checked against the answer key

Analyze the question We have an A.P. with first term a = -20 and fourth term T₄ = 37. We need the common difference d. Concept The nth term of an A.P. is: Tₙ = a + (n - 1)d Solution For the 4th term: T₄ = a + 3d 37 = -20 + 3d 3d = 37 + 20 = 57 d = 57 ÷ 3 = 19 Check: The terms are -20, -1, 18, 37. Each step adds 19, and the 4th term is 37. ✔ Option B (57) is a trap: it is the value of 3d, not d. Many students forget the final division by 3. Exam tip: In A.P. questions, always write the formula Tₙ = a + (n - 1)d first. Notice that the 4th term uses (n - 1) = 3, not 4. Then check your answer by listing the terms.

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