if a + b + c = 12 and a^ 2 + b^ 2 + c ^ 2 equal to 64 find the value of AB + BC + CA
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Answer:
a + b + c = 12
squaring on both sides :
(a + b + c)² = (12)²
a² + b² + c² + 2ab + 2bc + 2ca = 144
64 + 2(ab + bc + ca) = 144
2(ab + bc + ca) = 144 - 64
2(ab + bc + ca) = 80
ab + bc + ca = 80 ÷ 2
ab + bc + ca = 40
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