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`f(x) = x^2+2x+9`
`x^2+2x+9 = lambda`
`x^2+2x+9-lambda = 0`
If the above quadratic function does not have real roots then the discriminant(Delta) should be less than 0.
`Delta = 4-4xx1xx(9-lambda)<0`
So for `f(x) = lambda` where there is no solutions `lambda<8`
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