3 0 0 x 2 9 9 + 2 9 9 x 4 + 3 4 3 x 3 + 2 3 x + 3 0 0 = 0
If α is the real root of the equation above, find ⌊ α ⌋ .
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Let f ( x ) = 3 0 0 x 2 9 9 + 2 9 9 x 4 + 3 4 3 x 3 + 2 3 x + 3 0 0 . We note that: f ( 0 ) = 3 0 0 and f ( − 1 ) = − 6 7 . Therefore, there is a real root of f ( x ) in − 1 < x < 0 . And this is the only root because for x < − 2 or x > 2 , f ( x ) ≈ 3 0 0 x 2 9 9 a constantly increasing function. Therefore, − 1 < α < 0 ⇒ ⌊ α ⌋ = − 1 .
The following graph shows the real root α .
By observation, it is clear that the root is negative as the expression is positive for x>0 and equals 300 at x=0.Now by some hit and trial f(-1)<0 => f(0)*f(-1)<0=> The real root lies between -1 and 0 => [alpha]=-1
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We can do this by eradicating cases.
If α > 0 we see that whole expression always remains positive.
If α < − 1 we see that 3 0 0 x 2 9 9 becomes too much large for the expression to be zero , making the polynomial always negative.
So we can easily conclude that − 1 < α < 0 and hence ⌊ α ⌋ = − 1