The roots of a monic quadratic equation are 1 + 3 and 1 − 3 . Find the sum of its coefficients.
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.
Let the quadratic equation be a X 2 + b X + c
Let the two roots of this equation be X = r 1 and X = r 2 .
This can be written as: X − r 1 = 0 and X − r 2 = 0
Multiplying both sides: ( X − r 1 ) ( X − r 2 ) = 0
Multiplying out the brackets: X 2 − r 2 X − r 1 X + r 1 r 2 = 0
Simplified quadratic equation: X 2 − ( r 1 + r 2 ) X + ( r 1 r 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
Substituting (r_1+ r_2) and \( (r_1r_2) , the quadratic equation is: X 2 − ( 2 ) X + ( − 2 ) = X 2 − 2 X − 2
Comparing this with the quadratic equation at the top gives: a = 1 , b = − 2 , c = − 2
Therefore, a + b + c = 1 − 2 − 2 = − 3
Assuming he meant a monic polynomial. That is a = 1 and the quadratic is of the form x 2 + b x + c = 0
We can use Vieta's to calculate b and c .
b = − ( 1 + 3 + 1 − 3 ) = − 2 & c = ( 1 + 3 ) ( 1 − 3 ) = 1 − 3 = − 2
∴ a + b + c = 1 − 2 − 2 = − 3
Problem Loading...
Note Loading...
Set Loading...
S = 1 + 3 + 1 − 3 = 2
P = ( 1 + 3 ) ( 1 − 3 ) = − 2
Vieta!
x 2 − 2 x − 2 = 0 ⟹ a + b + c = − 3