An algebra problem by Sakanksha Deo

Algebra Level 4

For x 2 + 2 x + 5 x^{2}+ 2x + 5 to be a factor of x 4 + p x 2 + q x^{4 }+ p x^ {2} + q , the value of p and q should respectively be:

25,6 4,25 5,2 35,4 2,5 6,25

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2 solutions

The solution can be found using long division as follows:

We note that for x 2 + 2 x + 5 x^2+2x+5 to be a factor of x 4 + p x 2 + q x^4+px^2+q ,

{ 2 ( p 1 ) = 10 p 1 = 5 p = 6 q = 5 ( p 1 ) q = 5 × 5 q = 25 p , q = 6 , 25 \begin{cases} 2(p-1)=10 & \Rightarrow p-1 = 5 &\Rightarrow p = 6 \\ q = 5(p-1) & \Rightarrow q = 5\times 5 &\Rightarrow q = 25 \end{cases} \quad \Rightarrow p, q = \boxed {6,25}

Another way is to express the degree 4 polynomial as follows:

( x 2 + 2 x + 5 ) ( x 2 + l x + m ) = x 4 + p x 2 + q , for some constants l , m (x^2+2x+5)(x^2+lx+m)=x^4+px^2+q~,\textrm{ for some constants }l,m

Expanding the LHS and comparing coefficients of terms from both sides, it can easily be obtained that p = 6 , q = 25 p=6,q=25 .

Prasun Biswas - 6 years, 3 months ago

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It's a good idea.

Sai Ram - 5 years, 11 months ago

Can you plz give a detailed solution for this ...plzz.....

Sakanksha Deo - 6 years, 3 months ago

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Done that. It is better not to use objective answers if possible. Because, those who answer have only one chance, and it can be wrong because of computation errors, like I frequently do.

Chew-Seong Cheong - 6 years, 3 months ago

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I'll look to follow your advice in future...

Sakanksha Deo - 6 years, 3 months ago
Aareyan Manzoor
Mar 12, 2015

let the roots of x 2 + 2 x + 5 x^2+2x+5 be r 1 , r 2 r_1,r_2 . than the following must also be roots of x 4 + p x 2 + q x^4+px^2+q .it means r 1 4 + p r 1 2 + q = 0 r 2 4 + p r 2 2 + q = 0 \begin{aligned} r_1^4+pr_1^2+q=0\\ r_2^4+pr_2^2+q=0 \end{aligned} adding these 2, ( r 1 4 + r 2 4 ) + p ( r 1 2 + r 2 2 ) + 2 q = 0 (r_1^4+r_2^4)+p(r_1^2+r_2^2)+2q=0 by vieta's we have r 1 + r 2 = 2 , r 1 r 2 = 5 r_1+r_2=-2,r_1r_2=5 hence r 1 4 + r 2 4 = ( ( r 1 + r 2 ) 2 2 r 1 r 2 ) 2 2 r 1 2 r 2 2 = 14 r_1^4+r_2^4=((r_1+r_2)^2-2r_1r_2)^2-2r_1^2r_2^2=-14 r 1 2 + r 2 2 = ( r 1 + r 2 ) 2 2 r 1 r 2 = 6 r_1^2+r_2^2=(r_1+r_2)^2-2r_1r_2=-6 hence, 14 6 p + 2 q = 0 p = 3 q + 7 -14-6p+2q=0\rightarrow p=3q+7 the only ones from option satisfying this is 6 , 25 \boxed{6,25}

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