quadratic polynomials+equations

Algebra Level 2

If A and B are the roots of the quadratic equation x 2 x^2 - 4x + 1 = 0, the value of A 3 A^3 + B 3 B^3 is


The answer is 52.

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19 solutions

Analyzing the quadratic, we can see that A + B = 4 A + B = 4 and A B = 1 AB = 1 .

Since A 2 + B 2 = ( A + B ) 2 2 A B A^2 + B^2 = (A+B)^2 - 2AB and A 3 + B 3 = ( A 2 A B + B 2 ) ( A + B ) A^3 + B^3 = (A^2 - AB + B^2)(A+B) , we can simply plug the values to find out that A 2 + B 2 = 4 2 2 = 14 A^2 + B^2 = 4^2 - 2 = 14 and thus A 3 + B 3 = ( 14 1 ) × 4 = 52. A^3 + B^3 = (14 - 1) \times 4 = \boxed{52. \:}

how to analyze the quadratic

Saadat Shahidi - 7 years, 5 months ago

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For a quadratic equation a x 2 + b x + c = 0 ax^2+bx+c=0 , the sum of the roots is b a \frac{-b}{a} and product of the roots is c a \frac{c}{a} .

Guilherme Dela Corte - 7 years, 5 months ago

it easier than my way.

gousa lexy luqmana - 7 years, 5 months ago
Ajay Maity
Dec 18, 2013

For a quadratic equation a x 2 + b x + c = 0 ax^{2}+bx+c=0 , sum of the roots is b a \frac{-b}{a} and product of the roots is c a \frac{c}{a} .

So, here A + B = 4 and A*B = 1.

We need to calculate A 3 + B 3 A^{3} + B^{3} :

A 3 + B 3 = ( A + B ) ( A 2 + B 2 A B ) A^{3} + B^{3} = (A + B)(A^{2} + B^{2} - AB) ..... Identity of sum of cubes

= ( A + B ) ( ( A + B ) 2 2 A B A B ) (A + B)((A+B)^{2} - 2AB - AB) ..... Identity of sum of squares

Substitute the values of A + B and AB,

= (4)( 4 2 2 × 1 1 ) 4^{2} - 2\times1 - 1)

= 52 \boxed{52}

That's the answer!

NICE SOLUTION SIR ! CAN WE ALSO SOLVE IT BY QUADRATIC FORMULA?

Devesh Rai - 7 years, 5 months ago

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Or suppose if you solve the same sum by taking the quadratic equation: x 2 + x + 1 = 0 x^{2} + x + 1 = 0 . If you find its roots using the quadratic formula, it would lead into complex results. You would then have to find the cubes of those complex numbers.

This sum and product formulae would help here. I hope you are getting what I am trying to say!

Ajay Maity - 7 years, 5 months ago

You can solve it using that too. In this case, it would be easy to calculate the roots, in fact. But usually the method I wrote is preferred, as it can be used very easily for big quadratic equations too.

I suggest you remember these formulae. It would come in handy in lots of mathematical problems.

Ajay Maity - 7 years, 5 months ago

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OK SIR I GOT IT ! THANK YOU !!

Devesh Rai - 7 years, 5 months ago

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@Devesh Rai You are welcome!

Ajay Maity - 7 years, 5 months ago
Kshitij Johary
Apr 7, 2014

From the property of quadratic polynomials, it is clear that A + B = 4 A + B = 4 and A B = 1 AB = 1

( A + B ) 3 = A 3 + B 3 + 3 A B ( A + B ) (A + B)^3 = A^3 + B^3 + 3AB(A+B) .

Substituting the values and solving the equation for A 3 + B 3 A^3 + B^3 , we get the answer as 52 \boxed {52} .

Prasun Biswas
Dec 21, 2013

We have the equation x 2 4 x + 1 = 0 x^{2}-4x+1=0 with roots A and B.

We know, A + B = A+B= (-(coefficient of x))/(coefficient of x 2 ) x^{2}) = ( 4 ) 1 = 4 = \frac{-(-4)}{1} = 4

Also, A B = AB= (constant term of the polynomial)/(coefficient of x 2 ) x^{2}) = 1 1 = 1 = \frac{1}{1} = 1

Now, A 3 + B 3 = ( A + B ) ( A 2 A B + B 2 ) = ( A + B ) ( ( A + B ) 2 3 A B ) = 4 ( 4 2 3 × 1 ) = 4 ( 16 3 ) = 4 × 13 = 52 A^{3}+B^{3} = (A+B)(A^{2}-AB+B^{2}) = (A+B)((A+B)^{2}-3AB) = 4(4^{2}-3\times 1) = 4(16-3) = 4\times 13 =\boxed{52}

Sourav Maji
Dec 19, 2013

A and B are roots of eqn. x 2 x^{2} -4x+1=0

So, A+B= 4 / 1 {4/1}

AB=1

A 3 A^{3} + B 3 B^{3} = ( A + B ) 3 (A+B)^{3} -3 AB (A+B)

Putting value of A+B and AB we get A 3 A^{3} + B 3 B^{3} =52

Sushmita G
Dec 18, 2013

Since it is a quadratic equation,

The sum of the roots i.e. A+B= -(coefficient of x)/(coefficient of x^2)

Therefore, A+B= -(-4)= 4

The product of the roots i.e. A*B = (constant)/(coefficient of x^2)

Therefore, A*B = 1

Since we need A^3 + B^3 that is equal to (A + B)(A^2 -A*B + B^2), we need to find the value of A^2 + B^2.

We know that, (A + B)^2 = A^2 +2 A*B + B^2

Therefore, A^2 + B^2 = (A + B)^2 - 2 A*B

Putting the values, we get

A^2 + B^2 = (4)^2 - 2*1

= 16 - 2

= 14

Now, A^3 + B^3 = (A + B)(A^2 -A*B + B^2)

= (A + B)(A^2 + B^2 -A*B)

= 4(14 - 1)

= 4*13

= 52

A 3 + B 3 = ( A + B ) 3 3 A B ( A + B ) \color{#20A900}{A^3+B^3=(A+B)^3-3AB(A+B)} From Vieta's formulas we know that A + B = 4 , A B = 1 \color{#D61F06}{A+B=4,AB=1} .So A 3 + B 3 = 4 3 3 ( 1 ) ( 4 ) = 64 12 = 52 \color{#3D99F6}{A^3+B^3=4^3-3(1)(4)=64-12=\boxed{52}}

Vinayak Rastogi
Mar 1, 2014

according to the quadratic equation x=(-b±√(b^2-4ac))/2a. x = 2+3^1/2 (A) and 2-3^1/2 (B) . let them be equal to A and B. then solving by the identities (a+b)^3 = a^3 + b^3 + 3ab(a+b) and (a-b)^3 = a^3 - b^3 - 3ab(a-b) ,, we gt A^3 + B^3 = 52

If A and B are the roots of the given equation then A + B = 4 and A B =1 Then A^{3} + B^{3} = (A + B )^{3} - 3AB(A + B) = 4^{3} - 3 1*4 = 64 -12 = 52

Sandeep Sharma
Jan 16, 2014

IF P & Q ARE ROOTS OF A QUADRATIC EXPRESSION ax^2+bx+c=0 THEN P+Q=-b/a AND P*Q=c/a A+B= -b/a = - (- 4)/1 = 4 and AB = c/a = 1/1 = 1 hence, A^3 + b^3= (A+B)^3 - 3 AB (A+B) => 4^3 - 3 (1) (4) => 64 - 12 => 52

let A+B = -b/a and AB = c/a. and A^3 + b^3 = (A+B)^3 -3AB(A+B) so (4)^3 -3(1)(4) = 52

Sandeep Kumar
Jan 11, 2014

alpha + beta are the roots of quadratic equation ( class 9 th syllabus )

alpha + beta = -b/a = 4 alpha * beta = c/a = 1 (alpha + beta )^3 = alpha^3 + beta^3 + 3 alphta * beta ( alpha + beta ) [[a+b)^3 = a^3 + b^3 + 3ab(a+b)]]

=4^3 + 3 (1) (-4) = 64 -12

Saqib M
Dec 22, 2013

We know that if A and B are the roots of a quadratic ax^{2} + bx + c then, A+B= \frac{-b}{a} & AB= \frac{c}{a} . So, A+B=4 and AB=1. And we know that (A+B)^{3} = A^{3} + B^{3} + 3AB(A+B) or A^{3} + B^{3} = (A+B)^{3} - 3AB(A+B). Substituting the values of (A+B) and (AB) in the equation, A^{3} + B^{3} = 4^{3} - 3(1)(4) = 64-12 = \boxed{52}

Saad Ahmed
Dec 19, 2013

A=2+sqt3 B=2-sqt3 A^3+B^3=52

Hùng Minh
Dec 18, 2013

A and B are the roots of the quadratic equation x2 - 4x + 1 = 0 <=> A + B = 4, A x B = 1. A^3 + B^3 = (A + B)(A^2 - A x B + B^2) = (A + B)[(A + B)^2 - 3x A x B] = 4 x ( 4^2 - 3 x 1) = 52

Ritwik Verma
Dec 18, 2013

A+B= 4 AB=1 since, (A+B)^3= A^3 +B^3 + 3AB(A+B) this implies, A^3 + B^3 = (A+B)^3- 3AB(A+B) or A^3 + B^3 = 4^3 -3(4) or A^3 + B^3 = 52

Arvind Reddy
Dec 18, 2013

Sum of the roots A+B =-b/a = 4 and Product of the roots AB =c/a = 1

Then we use A^3 +B^3 = (A+B)^3 -2AB(A+B) = (4)^3 -3 1 4 = 52

The formula is A^3 +B^3 = (A+B)^3 -3 AB(A+B) = 4^3 -3 1*2 = 52.There was a typo earlier.I am sorry

Arvind Reddy - 7 years, 5 months ago
Fozley Abed
Dec 18, 2013

from given equation,we get A+B=4 and AB=1 ( as given roots are A and B ) ,So we get A^3+B^3= 4^3-3 1 4=52

Daniel Thompson
Dec 18, 2013

If A A and B B are the roots of the equation x 2 4 x + 1 = 0 x^2-4x+1=0 then the equation can be written as ( x A ) ( x B ) = 0 x 2 ( A + B ) x + A B = 0 (x-A)(x-B)=0 \iff x^2-(A+B)x+AB=0 . Therefore A + B = 4 A+B=4 .

That A 2 4 A + 1 = 0 A^2-4A+1=0 and B 2 4 B + 1 = 0 B^2-4B+1=0 is true since A A and B B are the roots of the equation. By addition, ( A 2 + B 2 ) 4 ( A + B ) + 2 = 0 (A^2+B^2)-4(A+B)+2=0 , so A 2 + B 2 = 4 ( A + B ) 2 = 14 A^2+B^2=4(A+B)-2=14 .

Also A 3 4 A 2 + A = 0 A^3-4A^2+A=0 and B 3 4 B 2 + B = 0 B^3-4B^2+B=0 , by addition again, ( A 3 + B 3 ) 4 ( A 2 + B 2 ) + ( A + B ) = 0 (A^3+B^3)-4(A^2+B^2)+(A+B)=0 , so A 3 + B 3 = 4 ( A 2 + B 2 ) ( A + B ) = 52 A^3+B^3=4(A^2+B^2)-(A+B)=52 .

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