50.
Ta có: `x:y:z = 3:4:5 => x/3 = y/4 = z/5 = k`
`=> x=3k; y = 4k => z = 5k`
`=> 2x^2 + 2y^2 - 3z^2 = -100`
`=> 2(3k)^2 + 2(4k)^2 - 3(5k)^2 = -100`
`=> 2.3^2 .k^2 + 2.4^2 .k^2 - 3.5^2 . k^2 = -100`
`=> 2.9 .k^2 + 2.16 .k^2 - 3.25 . k^2 = -100`
`=> 18 .k^2 + 32 .k^2 - 75 . k^2 = -100`
`=> k^2 (18+32-75) = -100`
`=> k^2 . (-25) = -100`
`=> k^2 = 4`
`=> k = 2` hoặc `-2`
`Th1: => x/3 = y/4 = z/5 = 2 => x=6;y=8;z=10`
`Th2: => x/3 = y/4 = z/5 = -2 => x=-6;y=-8;z=-10`
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