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Question : If (a + b + c) = 12 and (ab + bc + ca) = 47, find the value of (a+ b+ c– 3abc).

Option 1: 24

Option 2: 36

Option 3: 48

Option 4: 42


Team Careers360 23rd Jan, 2024
Answer (1)
Team Careers360 25th Jan, 2024

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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