If x2−yza=y2−zxb=z2−xyc, show that (x+y+z)(a+b+c)=ax+by+cz.
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Solution
x2−yza=y2−zxb=z2−xyc =x2−yz+y2−zx+z2−xya+b+c =x(x2−yz)+y(y2−zx)+z(z2−xy)ax+by+cz =x3+y3+z3−3xyzax+by+cz =(x+y+z)(x2+y2+z2−xy−yz−zx)ax+by+cz ∴(x+y+z)(a+b+c)=ax+by+cz, by (2) and (3).