Question : If (a + b + c) = 12 and (ab + bc + ca) = 47, find the value of (a3 + b3 + c3 – 3abc).
Option 1: 24
Option 2: 36
Option 3: 48
Option 4: 42
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Correct Answer: 36
Solution : Given: (a + b + c) = 12 and (ab + bc + ca) = 47. On squaring both sides of the equation, (a + b + c) = 12, we get, (a + b + c) 2 = 12 2 ⇒ a 2 + b 2 + c 2 + 2(ab + bc + ca) = 144 ⇒ a 2 + b 2 + c 2 + 2(47) = 144 ⇒ a 2 + b 2 + c 2 = 144 – 94 ⇒ a 2 + b 2 + c 2 = 50 The value of (a 3 + b 3 + c 3 – 3abc) is, a 3 + b 3 + c 3 – 3abc = (a + b + c)(a 2 + b 2 + c 2 – (ab + bc + ca)) ⇒ a 3 + b 3 + c 3 – 3abc = 12(50 – 47) ⇒ a 3 + b 3 + c 3 – 3abc = 12 × 3 $\therefore$ a 3 + b 3 + c 3 – 3abc = 36 Hence, the correct answer is 36.
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