sum of CoefficIent

Algebra Level pending

Given that, x 2 b x + c = 0 x^2 - bx + c = 0 and x = 2 + 1 2 1 x = \frac{ \sqrt{2} + 1}{ \sqrt{2} - 1}

Then, find the value of 1 + b + c 1 + b + c .


The answer is 8.

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2 solutions

Sajjad Sajjad
Mar 20, 2015

As, x = 2 + 1 2 1 x = \frac{ \sqrt{2} + 1}{ \sqrt{2} - 1}

We get, x = 3 + 2 2 x = 3 + 2 \sqrt{2}

As, a x 2 b x + c = 0 ax^2 - bx + c = 0 is a Binomial Equation, there have another root, x = 3 2 2 x = 3 - 2 \sqrt{2}

So, ( x 3 2 2 ) ( x 3 + 2 2 ) = 0 (x - 3 - 2 \sqrt{2})(x - 3 + 2 \sqrt{2})=0

At last, x 2 6 x + 1 = 0 x^2 - 6x + 1 = 0

Compairing with, a x 2 b x + c = 0 ax^2 - bx + c = 0 we get, a = 1 , b = 6 , c = 1 a=1, b=6, c=1

So, a + b + c = 1 + 6 + 1 = 8 a + b + c = 1 + 6 + 1 = \boxed{8}

I think, the question shouldn't be for only 10 points... But, what to do? :)

Note that the equation 2 x 2 12 x + 2 = 0 2 x^2 - 12x + 2 = 0 also has your given value as a root, hence your answer was not uniquely determined. I have added the condition that a = 1 a = 1 .

Calvin Lin Staff - 6 years, 2 months ago

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I get it... Thanks, Calvin bro... :)

Sajjad Sajjad - 6 years, 2 months ago

When you post a problem, if you set a level that will make it be worth 100 points, until we announce a level.

Calvin Lin Staff - 6 years, 2 months ago
Tijmen Veltman
Mar 19, 2015

x = 2 + 1 2 1 2 + 1 2 + 1 = ( 2 + 1 ) 2 = 3 + 2 2 = b 2 a ± b 2 4 a c 2 a x=\frac{\sqrt{2}+1}{\sqrt{2}-1}\frac{\sqrt{2}+1}{\sqrt{2}+1} =(\sqrt{2}+1)^2=3+2\sqrt{2}=\frac{b}{2a}\pm\frac{\sqrt{b^2-4ac}}{2a} .

This gives b = 6 a b=6a and 8 = b 2 4 a c ( 2 a ) 2 = 9 c a 8=\frac{b^2-4ac}{(2a)^2}=9-\frac{c}{a} , meaning that c = a c=a . All in all this gives us:

a x 2 b x + c = a ( x 2 6 x + 1 ) ax^2-bx+c=a(x^2-6x+1)

and a + b + c = a + 6 a + a = 8 a a+b+c=a+6a+a=\boxed{8a} . This is valid for any a R a\in\mathbb{R} , hence as of yet this problem has no single solution. With the (in a way logical) choice of a = 0 a=0 however, we obtain a + b + c = 8 a+b+c=\boxed{8} .

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