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Cubic Root Comparison (Posted on 2023-07-05) Difficulty: 3 of 5
Let P(x)=x3-4x2+39x-46 and Q(x)=x3+3x2+4x-3 be two polynomials.

1. Prove that each of P, Q has an unique real root. Let them be a, b respectively.
2. Prove that {a}>{b}2, where {x}=x-[x] is the fractional part of x.

No Solution Yet Submitted by Danish Ahmed Khan    
Rating: 5.0000 (1 votes)

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Solution well it is a proof if you just solve it | Comment 1 of 2
syms x
fplot(@(x)x^3-4*x^2+39*x-46)
hold on
fplot(@(x)x^3+3*x^2+4*x-3)
hold off
grid
r=roots([1,-4,39,-46])
r2=roots([1,3,4,-3])

produces a graph showing each of the two functions crossing the x axis once (and each other twice--almost tangent), and then listing roots:

r =
           1.35203442644909 +      5.80238880628641i
           1.35203442644909 -      5.80238880628641i
           1.29593114710183 +                     0i
r2 =
          -1.75799011384641 +      1.65034755068945i
          -1.75799011384641 -      1.65034755068945i
          0.515980227692821 +                     0i
          
the real roots being, of course, those whose imaginary component is zero.

The fractional part of a is .29593114710183.
The fractional part of b is .515980227692821, which is in fact b itself, whose square is  0.266235595369935.

So the fractional part of a is indeed larger than the square of the fractional part of b.

  Posted by Charlie on 2023-07-06 17:41:53
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