WAEC Mathematics 2025 Theory — Question 3
Question 3 of 19 from the West African Examinations Council (WAEC) Mathematics 2025 Theory paper, with the correct answer and a full explanation.
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3. A company installs solar panels to reduce electricity cost. Monthly savings S is modelled by S = 200 + 50x − 2x², where x is the number of months after installation. (a) At what time will the savings on electricity stop increasing? (b) Find the maximum savings.
Model answer
(a) dS/dx = 50 − 4x = 0 → x = 12.5 months (b) S = 200 + 50(12.5) − 2(12.5)² = 200 + 625 − 312.5 = $512.50 per month
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