1. `3x^2-6x+5=(\sqrt3x)^2 -2.\sqrt3x .\sqrt3 +(\sqrt3)^2 +2`
`=(\sqrt3x-\sqrt3)^2+2>0`
2. `-x^2+2x-7=-(x^2-2x+7)`
`=-(x^2-2.x.1+1^2+6)`
`=-(x-1)^2-6<0`
3. `-5x^2+20x-49=-(5x^2-20x+49)`
`=-( (\sqrt5x)^2 -2.\sqrt5x .2\sqrt5 + (2\sqrt5)^2 + 29)`
`=-(\sqrt5x-2\sqrt5)^2-29<0`