Maths Olympiad Prep

Library / /450 of 520

Algebra Difficulty 6.1 National olympiad Prove it

Does the equation

27x4+18x3x+1=0 27 x^{4}+18 x^{3}-x+1=0

have real roots?

Solution

Solution: The necessary and sufficient condition for a quartic equation to be reducible to a quadratic equation, as established in problem 656 (October 1955 issue, p. 57), is satisfied in this case. Indeed,

a=1827=23,b=0,c=127 a=\frac{18}{27}=\frac{2}{3}, \quad b=0, \quad c=-\frac{1}{27}

and thus

a34ab+8c=827827=0 a^{3}-4 a b+8 c=\frac{8}{27}-\frac{8}{27}=0

Using the transformation x=za4=z16x=z-\frac{a}{4}=z-\frac{1}{6} given there, we obtain the equation

(z16)4+23(z16)3127(z16)+1=z416z2+13011296=0 \left(z-\frac{1}{6}\right)^{4}+\frac{2}{3}\left(z-\frac{1}{6}\right)^{3}-\frac{1}{27}\left(z-\frac{1}{6}\right)+1=z^{4}-\frac{1}{6} z^{2}+\frac{1301}{1296}=0

The discriminant of this equation, which is quadratic in z2z^{2}, is negative, and thus it cannot have real roots.

Second solution: Notice that if we multiply our equation by 12 and add and subtract 36x236 x^{2} to the left side, the left side can be completed to a perfect square:

324x4+216x3+36x236x212x+1=11 324 x^{4}+216 x^{3}+36 x^{2}-36 x^{2}-12 x+1=-11

or

(18x2+6x1)2=11 \left(18 x^{2}+6 x-1\right)^{2}=-11

The left side cannot be negative for real xx, while the right side is negative, so our equation cannot be satisfied by any real root.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.