If a2 + b2 + c2 = 576 and (ab + bc + ca) = 50 , then what is the value of (a + b + c), if (a+b+c) < 0 ? [2Marks]
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Solution
(a + b + c)2 = a2 + b2 +c2 + 2 (ab + bc + ca)
(a + b + c)2 =576 + 2 x 50 [1Mark]
(a + b + c)2 = 676
a + b + c = 26 or -26
a + b + c < 0
so a+b+c = -26 [1Mark]