Thursday

January 29, 2015

January 29, 2015

Posted by **Drake** on Wednesday, June 26, 2013 at 10:41am.

{1+(a+b)^2}x^2−2(2a+2b+1)x+5=0

has real roots, what is the value of

a^3+b^3+6ab+a+b?

- Math (Quadratics) -
**Steve**, Wednesday, June 26, 2013 at 11:25amthe discriminant must not be negative, so

4(2a+2b+1)^2 - 4*5(1+(a+b)^2) >= 0

a^2 + 2ab + b^2 - 4(a+b) + 4 >= 0

(a+b)^2 - 4(a+b) + 4 >= 0

((a+b)-2)^2 >= 0

Hmmm. That's true for any a,b.

If a+b=2, then the original equation has a repeated root and

a^3+b^3+6ab+a+b = (a+b)(a^2-ab+b^2) + (a+b) + 6ab

= (a+b)(a^2+b^2+1) + ab(6-(a+b))

= 2(a^2+b^2+1) + ab(6-2)

= 2a^2+2b^2+2 + 4ab

= 2(a^2+2ab+b^2)+2

= 2(a+b)^2 + 2

= 2*4+2

= 10

Haven't yet worked it out for other a,b but the wording of the question makes me believe the expression is 10 for any a,b.

**Answer this Question**

**Related Questions**

Math - Determine for what value(s) of d the quadratic equation 5x^2-10x+d = 0 ...

Precalculus - "Show that x^6 - 7x^3 - 8 = 0 has a quadratic form. Then find the ...

Math (Algebra) - If the ratio of the roots of the quadratic equation x^2−...

Maths - There are two complex roots to the quadratic equation z^2 +(1−16i)...

Algebra 2(quadratic) - Use the value of the discriminant to determine the number...

maths - If the quadratic equation (8+i)x^2+(a-i)x+38-2i=0 has real roots, what ...

Math (Quadratics)(Discriminants) - If a is a real number and a cubic equation f(...

Algebra2 - Find the polynomials roots to each of the following problems: #1) x^2...

math - I'm having trouble understanding one step in a proof of the Cauchy-...

Algebra 2 - I am trying to factor a 4th degree polynomial that does not have any...