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Algebra Level 3

The roots of a monic quadratic equation are 1 + 3 1+\sqrt3 and 1 3 1-\sqrt3 . Find the sum of its coefficients.


The answer is -3.

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4 solutions

Alan Yan
Aug 24, 2015

S = 1 + 3 + 1 3 = 2 S = 1 + \sqrt{3} + 1 - \sqrt{3} = 2

P = ( 1 + 3 ) ( 1 3 ) = 2 P = (1+\sqrt{3})(1 - \sqrt{3}) = -2

Vieta!

x 2 2 x 2 = 0 a + b + c = 3 x^2 -2x -2 = 0 \implies a+b+c = \boxed{-3}

Stuti Malik
Aug 24, 2015

Let the quadratic equation be a X 2 + b X + c aX^2 + bX+ c

Let the two roots of this equation be X = r 1 X=r_1 and X = r 2 X=r_2 .

This can be written as: X r 1 = 0 X-r1= 0 and X r 2 = 0 X-r2= 0

Multiplying both sides: ( X r 1 ) ( X r 2 ) = 0 (X-r1)(X-r2)=0

Multiplying out the brackets: X 2 r 2 X r 1 X + r 1 r 2 = 0 X^2 -r_2X- r_1X + r_1r_2= 0

Simplified quadratic equation: X 2 ( r 1 + r 2 ) X + ( r 1 r 2 ) X^2- (r_1+r_2) X + (r_1r_2)

Where, r_1= 1+ \sqrt{3} and \(r_2= 1- \sqrt{3}

Therefore:

[(r 1 + r 2 )= (1+ \sqrt{3}) + (1- root(3))= 2\ r 1 × r 2 ) = ( 1 + 3 ) ( 1 3 ) = 1 3 = 2 r_1\times r_2)=(1+ \sqrt{3})*(1- \sqrt{3}) = 1- 3= -2

Substituting (r_1+ r_2) and \( (r_1r_2) , the quadratic equation is: X 2 ( 2 ) X + ( 2 ) = X 2 2 X 2 X^2- (2)X+ (-2)=X^2-2X-2

Comparing this with the quadratic equation at the top gives: a = 1 , b = 2 , c = 2 a= 1, b= -2 , c=-2

Therefore, a + b + c = 1 2 2 = 3 a+b+c = 1 - 2- 2=\boxed{ -3}

Isaac Buckley
Aug 24, 2015

Assuming he meant a monic polynomial. That is a = 1 a=1 and the quadratic is of the form x 2 + b x + c = 0 x^2+bx+c=0

We can use Vieta's to calculate b b and c c .

b = ( 1 + 3 + 1 3 ) = 2 & c = ( 1 + 3 ) ( 1 3 ) = 1 3 = 2 b=-(1+\sqrt3+1-\sqrt3)=-2\, \, \&\, c=(1+\sqrt3)(1-\sqrt3)=1-3=-2

a + b + c = 1 2 2 = 3 \therefore a+b+c=1-2-2=\boxed{-3}

Jerson Suplemento
Aug 25, 2015

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