root of b , e , a , r b,e,a,r , poor bear!

Algebra Level 3

If b , e , a , r b,e,a,r are roots of P ( x ) = x 4 2 x 3 7 x 2 2 x + 1 = 0 P(x)=x^4-2x^3-7x^2-2x+1=0 . Find 1 b + 1 e + 1 a + 1 r \frac{1}{b}+\frac{1}{e}+\frac{1}{a}+\frac{1}{r}


The answer is 2.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Paola Ramírez
Jan 8, 2015

As b , e , a , r b,e,a,r are root of P ( x ) = x 4 2 x 3 7 x 2 2 x + 1 = 0 P(x)=x^4-2x^3-7x^2-2x+1=0 we can write :

P ( x ) = ( x b ) ( x e ) ( x a ) ( x r ) P(x)=(x-b)(x-e)(x-a)(x-r) P ( x ) = x 4 ( b + e + a + r ) x 3 + ( b e + b a + b r + e a + e r + a r ) x 2 ( b e a + b e r + e a r + b a r ) x + b e a r P(x)=x^4-(b+e+a+r)x^3+(be+ba+br+ea+er+ar)x^2-(bea+ber+ear+bar)x+bear

b + e + a + r = 2 b+e+a+r=2

b e + b a + b r + e a + e r + a r = 7 be+ba+br+ea+er+ar=-7

b e a + b e r + e a r + b a r = 2 bea+ber+ear+bar=2

b e a r = 1 bear=1

1 b + 1 e + 1 a + 1 r = b e a + b e r + e a r + b a r b e a r = 2 1 \frac{1}{b}+\frac{1}{e}+\frac{1}{a}+\frac{1}{r}=\frac{bea+ber+ear+bar}{bear}=\frac{2}{1}

1 b + 1 e + 1 a + 1 r = 2 \boxed{\frac{1}{b}+\frac{1}{e}+\frac{1}{a}+\frac{1}{r}=2}

Sujoy Roy
Jan 8, 2015

Put x = 1 y x=\frac{1}{y} in the given equation we get, y 4 2 y 3 7 y 2 2 y + 1 = 0 y^4-2y^3-7y^2-2y+1=0 whose roots are 1 b , 1 e , 1 a , 1 r \frac{1}{b}, \frac{1}{e}, \frac{1}{a}, \frac{1}{r} .

So, 1 b + 1 e + 1 a + 1 r = 2 \frac{1}{b}+\frac{1}{e}+\frac{1}{a}+\frac{1}{r}=\boxed{2} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...