If a+b+c=6 and a^{3}+b^{3}+c^{3}-3 a b c=126, then a b+ {bc}+ {ca} is equal to:

SSC CGL 12 Jun 2019 Shift 2 Paper

Question : 66 of 100

Marks: +1, -0

a+b+c=6

and

a^{3}+b^{3}+c^{3}-3 a b c=126,

then

a b+

\ {bc}+\ {ca}

is equal to:

Solution:

Put

c=0

a+b=6

a^{3}+b^{3}=126

a^{3}+b^{3}=(a+b)\ (a^{2}+b^{2}-a b\ )

126=(6)\ ((a+b)^{2}-3 a b\ ) )

21=36-3 a b

a b=\ {15}/{3}=5

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