Relations between the Roots and the Coefficients of a Quadratic Equation.
How to find the relation between roots and the coefficients of a quadratic equation.
Let us the quadratic equation of the general form
ax2+bx+c=0 where a does not equal zero. a is a coefficient of x2,b is a coefficient of x, and c, is the constant term.
Now,
The standard form of a quadratic equation is
ax2+bx+c=0 where aโ 0
Divided the equation by a
X2+bx/a +c/a=0
Take constant terms to the R.H.S
X2+bx/a=-c/a
To complete the square on the L.H.S add (b/2a)2 to both sides.
X2+bx/a+ (b/2a)2=(b/2a)2-c/a
(x+b/2a)2=b2/4a2-c/a
(x+b/2a)2= b2-4ac/4a2
Square roots on both side
x+b/2a=ยฑโ(b^2-4ac/2a
x=-b/2aยฑโ(b^2-4ac/2a
x=(-bยฑโ(b^2-4ac)/2a
Hence the solution of quadratic ax2+bx+c=0 where aโ 0 is given by
x=-bยฑโ(b^2-4ac/2a
Let ฮฑ and ฮฒ be the roots of ax2+bx+c=0 where aโ 0 such that.
ฮฑ=-b+โ(b^2-4ac/2a and ฮฒ=-b-โ(b^2-4ac/2a
Therefore,
ฮฑ+ฮฒ=(-b+โ(b^2-4ac/2a ) + (-b-โ(b^2-4ac/2a)
ฮฑ+ฮฒ=-b+โ(b^2-4ac- b-โ(b^2-4ac/2a
ฮฑ+ฮฒ=-2b/2a
ฮฑ+ฮฒ=-b/a
Sum of the roots =S=-b/a
Sum of the roots =S=-coefficient of x/coefficient of x2
Again,
ฮฑฮฒ=(-b+โ(b^2-4ac/2a)( -b-โ(b^2-4ac/2a )
ฮฑฮฒ =(-b+โ(b^2-4ac)( -b-โ(b^2-4ac)/4a2
ฮฑฮฒ=(-b)2-(โ(b^2-4ac)2/4a2
ฮฑฮฒ =b2-b2+4ac/4a2
ฮฑฮฒ=4ac/4a.a
ฮฑฮฒ= c/a
Product of the roots =P=c/a
Product of the roots =P=constant term /coefficient of x2
The above results are helpful in expressing symmetric functions of the roots in terms of the coefficients of the quadratic equations.
Therefore, Sum of the roots (ฮฑ+ฮฒ =-coefficient of x/coefficient of x2) and the product of the roots (ฮฑฮฒ=constant term/coefficient of x2) represent the required relations between roots (ฮฑ andฮฒ) and coefficients (a, b and c) of equation ax2+bx+c=0
For example:- if the roots of the equation
5x2-4x+7=0 be ฮฑ and ฮฒ, than
Sum of roots =-coefficient of x/coefficient of x2
Sum of roots =-(-4)/5
Sum of roots=4/5
Product of roots=constant term/coefficient of x2
Product of roots=7/5
Sum of Roots of a quadratic equation:-
The sum of the roots of a quadratic equation is equal to the negation of the coefficient of x divided by the coefficient of x2.it is denoted by S.
Sum of roots =S=-b/a
Sum of roots= -coefficient of x/coefficient of x2
For example:- Sum of roots of quadratic equation
X2-5x+6=0
Compare the standard form of quadratic equation
ax2+bx+c=0 where aโ 0
a=1
b=-5
c=6
Sum of roots =S=-b/a=-(-5)/1=5
Product of Roots of a quadratic equation:-
The products of the roots of a quadratic equation are equal to the constant term divided by the coefficient of x2.it is denoted by P.
Product of roots =P=c/a
Product of roots =constant term /coefficient of x2
For example:-Product of roots of quadratic equation
2x2+6x+3=0
Compare the standard form of quadratic equation
ax2+bx+c=0 where aโ 0
a=2
b=6
c=3
Product of roots=c/a