It is known that $x + y =15$ and $y + z = 10$ , simplify $6(x^2+y^2+z^2) -15xy + 20yz – 13zx$ ?

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Answer 1 · 2017-06-19T09:57:06

$0$

Solving

${(x+y=15),(y+z=10):}$

we have

$y = 15-x$ and $z=x-5$

Substituting into $6(x^2+y^2+z^2) -15xy + 20yz – 13zx$ we obtain

$6(x^2+y^2+z^2) -15xy + 20yz – 13zx=0$

It is known that x + y =15x + y =15 and y + z = 10y + z = 10, simplify 6(x^2+y^2+z^2) -15xy + 20yz – 13zx6(x^2+y^2+z^2) -15xy + 20yz – 13zx?