Dare to find my roots!

Algebra Level 5

x 5 + 5 λ x 4 x 3 + ( λ α 4 ) x 2 ( 8 λ + 3 ) x + ( λ α 2 ) = 0 λ , α R x^5+5\lambda x^4-x^3+(\lambda \alpha -4)x^2-(8\lambda+3)x+(\lambda \alpha -2)=0 \\ \lambda,\alpha \in \mathbb{R} If the value of α \alpha for which the above equation has exactly one root independent of λ \lambda is in the form a b -\dfrac{a}{b} where a a and b b are coprime positive integers , find a + b a+b .


The answer is 69.

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1 solution

Rishabh Jain
Jul 13, 2016

Let the expression be f(x). Write it as g ( x ) + λ p ( x ) g(x)+\lambda p(x) and since roots are independent of λ \lambda we must have g ( x ) = p ( x ) = 0 g(x)=p(x)=0 (at same value of x x ) so that f ( x ) = 0 f(x)=0 irrespective of λ \lambda .

f ( x ) = ( x 5 x 3 4 x 2 3 x 2 g ( x ) ) + λ ( 5 x 4 + α x 2 8 x + α p ( x ) ) = 0 f(x)=(\underbrace{x^5-x^3-4x^2-3x-2}_{\color{#D61F06}{g(x)}})+\lambda (\underbrace{5x^4+\alpha x^2-8x+\alpha}_{\color{#20A900}{p(x)}})=0

g ( x ) = 0 ( x 2 ) ( x 4 + 2 x 3 + 3 x 2 + 2 x + 1 ) = 0 ( x 2 ) ( x 2 + x + 1 ) 2 = 0 \color{#D61F06}{g(x)}=0\\\implies (x-2)(x^4+2x^3+3x^2+2x+1)=0\\\implies (x-2)(x^2+x+1)^2=0

Since f ( x ) f(x) has only one solution i.e x = 2 x=2 . Now since x = 2 x=2 setting p ( 2 ) = 0 p(2)=0 we get α = 64 5 \alpha =\dfrac{-64}{5} . Hence 64 + 5 = 69 \Large 64+5=\boxed{69} .

Hence f ( x ) f(x) has only one root and independent of λ \lambda f ( x ) f(x) vanishes at x = 2 x=2 when α = 64 5 \alpha=\dfrac{-64}{5} .

Great solution! (+1)

Akshat Sharda - 4 years, 11 months ago

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Thanks... :-)

Rishabh Jain - 4 years, 11 months ago

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