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  1. If   a + b + c = 0 then the value of
    a2 + b2 + c2
    		 is
    ab + bc + ca
    1. 2
    2. –2
    3. 0
    4. 4
Correct Option: B

a + b + c = 0
∴  (a + b + c)2 = 0
⇒  a2 + b2 + c2 + 2ab + 2bc + 2ca = 0
⇒  a2 + b2 + c2 = – 2ab – 2bc – 2ca

∴ 
a2 + b2 + c2
ab + bc + ca

=
−2(ab + bc + ca)
 = −2
ab + bc + ca


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