d) If (5a - 2b) = 7 and ab = 12, then find the value of (125a - 86°)
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Step-by-step explanation:
(5a -2b) ² = (5a)² +(2b) ² -2(5a)(2b)
7² = (5a)² +(2b) ² -20 ab
49 = (5a+2b) ² - 240
(5a)² +(2b) ² = 389
((5a)³ - (2b)³) = (5a-2b) × ( (5a)² + (5a)(2b) + (2b)² )
125a³ - 8b³ = 7 × (389 +120)
= 7× 509 = 3563
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