Question 21

If (a + b + c) = 6 and $$a^2 + b^2 + c^2$$ = 14, then (ab + bc + ca) = ?

Solution

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$$(a + b + c)^2 =a^2 + b^2 + c^2 + 2(ab + bc + ca)$$

$$(6)^2 =14 + 2(ab + bc + ca)$$

36-14 = 2(ab + bc + ca)

(ab + bc + ca) = 11

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