Bi-quadratics aren't easy

Algebra Level 4

( 12 x 1 ) ( 6 x 1 ) ( 4 x 1 ) ( 3 x 1 ) = 5 \large (12x -1) (6x - 1) (4x - 1) (3x - 1) = 5

If the product of the real roots of the above equation can be expressed as a b \dfrac{a}{b} , where a < 0 a < 0 and b > 0 b > 0 , and a a , b b are coprime,

then find a + b |a| + b .


The answer is 25.

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1 solution

We can multiply the first term with the fourth term, and the second one with the third one:

( 36 x 2 15 x + 1 ) ( 24 x 2 10 x + 1 ) = 5 (36x^2-15x+1)(24x^2-10x+1)=5

( 3 ( 12 x 2 5 x ) + 1 ) ( 2 ( 12 x 2 5 x ) + 1 ) = 5 (3(12x^2-5x)+1)(2(12x^2-5x)+1)=5

Now, let y = 12 x 2 5 x y=12x^2-5x , so:

( 3 y + 1 ) ( 2 y + 1 ) = 5 6 y 2 + 5 y 4 = 0 ( 3 y + 4 ) ( 2 y 1 ) = 0 y 1 = 4 3 y 2 = 1 2 (3y+1)(2y+1)=5 \\ 6y^2+5y-4=0 \\ (3y+4)(2y-1)=0 \\ y_1=-\dfrac{4}{3} \\ y_2=\dfrac{1}{2}

From here we have two solutions for y y , so there are two cases:

12 x 2 5 x = 4 3 36 x 2 15 x + 4 = 0 12x^2-5x=-\dfrac{4}{3} \\ 36x^2-15x+4=0

The discriminant is Δ = ( 15 ) 2 4 ( 36 ) ( 4 ) = 351 < 0 \Delta=(-15)^2-4(36)(4)=-351<0 , hence we don't have real roots.

The second case is:

12 x 2 5 x = 1 2 24 x 2 10 x 1 = 0 12x^2-5x=\dfrac{1}{2} \\ 24x^2-10x-1=0

The discriminant is Δ = ( 10 ) 2 4 ( 24 ) ( 1 ) = 196 > 0 \Delta=(-10)^2-4(24)(-1)=196>0 , so there are two distinct real solutions, and their product, by Vieta's formulas is 1 24 -\dfrac{1}{24} .

Comparing we get a = 1 a=-1 and b = 24 b=24 , thus the answer is a + b = 25 |a|+b=\boxed{25} .

Nice one, bro. Did it the same way.

Vishwak Srinivasan - 5 years, 9 months ago

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Cool . Nice. Excellent approach. All adjectives to you!

Aakash Khandelwal - 5 years, 9 months ago

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Oh, thank you :3

Alan Enrique Ontiveros Salazar - 5 years, 9 months ago

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