SBI PO Prelims practice question
If x = 3 sec A and y = 9 tan A, then the value of 81x² − 9y² is:
From a SBI PO Prelims Quantitative Aptitude practice set · Quantitative Aptitude
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Correct answer: (D) 729
x/3 = sec A, y/9 = tan A. Using sec²A − tan²A = 1: x²/9 − y²/81 = 1 ⇒ 81x² − 9y² = 9 × 81 = 729. (Adding instead of subtracting — using sec² + tan² — is the trap.)
हिंदी में प्रश्न
यदि x = 3 sec A और y = 9 tan A है, तो 81x² − 9y² का मान है:
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सही उत्तर: (D) 729
x/3 = sec A, y/9 = tan A। sec²A − tan²A = 1 से: x²/9 − y²/81 = 1 ⇒ 81x² − 9y² = 9 × 81 = 729। (घटाने के बजाय जोड़ना — sec² + tan² लेना — गलत है।)
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