Question

a^2+b^2+c^2=250
ab+bc+ca=3
Then a+b+c=?

Answer 1

(a+b+c)²=a²+b²+c²+2(ab+bc+ca)
or,(a+b+c)²=250+2*6
or,(a+b+c)²=256
or,a+b+c=√256
or,a+b+c=+16 or -16.
Ans:a+b+c is +16 and/or -16

Answer 2

We know that
  (a + b + c)² = (a² + b² + c²) + 2 (a b +  b c +  c a)
                   = 250 + 2 ( 3)
                   = 256

It means that  a + b + c = +16  or  - 16
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