Any smart ways for this?

Algebra Level 3

Find the integer root of this equation

4 x 3 24 x 2 + 23 x + 18 = 0 4{ x }^{ 3 }-24{ x }^{ 2 }+23x+18=0


The answer is 2.

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1 solution

Short Solution: \textbf{Short Solution:} Just apply the rational root test.

Long Solution: \textbf{Long Solution:} First depress the cubic by making the substitution x = y b 3 a = y + 24 12 = y + 2 x=y-\frac{b}{3a}=y+\frac{24}{12}=y+2 to get: 4 y 3 25 y = 0 4y^3-25y=0 Which can be factored as y ( 4 y 2 25 ) = y ( 2 y + 5 ) ( 2 y 5 ) = 0 y = 0 , 5 2 , 5 2 y(4y^2-25)=y(2y+5)(2y-5)=0\rightarrow y={0,-\frac{5}{2},\frac{5}{2}} .So the integer solution of the original cubic is x = 0 + 2 = 2 x=0+2=\boxed{2}

I used long solution

Aman Sharma - 6 years, 5 months ago

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