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This question looks harder than it appears (quadratic)

Given that A and B are roots of the equation x^2+3x-6=0, find a quadratic equation with integer coefficients whose roots are 2/A and 2/B.

This is what I done so far:

A=3+(33)2,B=3(33)2A= \frac{-3+\sqrt(33)}{2} , B=\frac{-3-\sqrt(33)}{2}

ax2+bx+c=0ax^2+bx+c=0

x1=4(33)3,x2=4(33)3x_1=\frac{4}{\sqrt(33)-3} , x_2=\frac{4}{-\sqrt(33)-3}

Unparseable latex formula:

\frac{4}(\sqrt(33)-3}+{4}{-\sqrt(33)-3}=\frac{-b}{a}


4(33)3×4(33)3=ca\frac{4}{\sqrt(33)-3} \times \frac{4}{-\sqrt(33)-3}=\frac{c}{a}

Forgot what to do now
(edited 11 years ago)
Reply 1
The question use sum and product surely

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

2/a * 2/b = 4/ab

From the original

a+b = -3
ab = -6
(edited 11 years ago)
Original post by Prokaryotic_crap
...


Your method of solving it is harder than it needs to be.

Recall a quadratic can be written as x^2-("sum of roots") x + "product of roots"=0

So, A+B = ..., AB=...

Now do the sum and product of roots for your new quadratic.
Reply 3
How about you just expand out (x2/A)(x2/B) (x - 2/A)(x - 2/B) ?
Reply 4
Try supposing that your expression is equal to (X-A)(X-B) and see what you can discover.
You don't need to solve A and B.

You should know facts about the roots of polynomials.

A+B=baA+B=-\frac{b}{a}

AB=caAB=\frac{c}{a}

So write down A+B and AB.

Now consider 2A+2B\frac{2}{A}+\frac{2}{B}

Can you rewrite this using A+B and AB?

Finally consider 2A×2B\frac{2}{A} \times \frac{2}{B}

Rewrite this using AB.

Now for your final answer. The quadratic is:

x2(2A+2B)x+(2A×2B)=0x^2 -(\frac{2}{A}+\frac{2}{B})x+ (\frac{2}{A} \times \frac{2}{B})=0

You will need to multiply the whole equation by 3 to obtain integer coefficients.
(edited 11 years ago)
Original post by Mr M
You don't need to solve A and B.

You should know facts about the roots of polynomials.

A+B=baA+B=-\frac{b}{a}

AB=caAB=\frac{c}{a}

So write down A+B and AB.

Now consider 2A+2B\frac{2}{A}+\frac{2}{B}

Can you rewrite this using A+B and AB?

Finally consider 2A×2B\frac{2}{A} \times \frac{2}{B}

Rewrite this using AB.

Now for your final answer. The quadratic is:

x2(2A+2B)x+(2A×2B)x^2 -(\frac{2}{A}+\frac{2}{B})x+ (\frac{2}{A} \times \frac{2}{B})

You will need to multiply the whole equation by 3 to obtain integer coefficients.


ok, this is what I got:

A+B=3,AB=6A+B= -3,AB=-6

2A+2B=2B+2AAB=2(A+B)AB=2(3)(6)=1\frac{2}{A}+\frac{2}{B}=\frac{2B+2A}{AB}=\frac{2(A+B)}{AB}=\frac{2(-3)}{(-6)}=1

2A×2B=4AB=4(6)=2(3)\frac{2}{A} \times \frac{2}{B}=\frac{4}{AB}=\frac{4}{(-6)}=\frac{2}{(-3)}

x2+x23=0x^2+x-\frac{2}{3}=0

3x2+3x2=03x^2+3x-2=0

correct?
(edited 11 years ago)
Original post by Prokaryotic_crap
ok, this is what I got:

A+B=3,AB=6A+B= -3,AB=-6

2A+2B=2B+2AAB=2(A+B)AB=2(3)(6)=1\frac{2}{A}+\frac{2}{B}=\frac{2B+2A}{AB}=\frac{2(A+B)}{AB}=\frac{2(-3)}{(-6)}=1

2A×2B=4AB=4(6)=2(3)\frac{2}{A} \times \frac{2}{B}=\frac{4}{AB}=\frac{4}{(-6)}=\frac{2}{(-3)}

x2+x23=0x^2+x-\frac{2}{3}=0

3x2+3x2=03x^2+3x-2=0

correct?


Look at the coefficient of x.

Remember the sum of the roots is =ba=-\frac{b}{a}
(edited 11 years ago)
Reply 8
Original post by Mr M
Look at the coefficient of x.

Remember the sum of the roots is =ba=- \frac{b}{a}


Ftfy too late
Reply 9
Did anyone mention the substitution method?

If x^2+3x-6=0 has roots A and B then (2/u)^2+3(2/u)-6=0 has roots 2/A and 2/B.

Multiply through by u^2 to finish.
Original post by BabyMaths
...


Very nice - PRSOM.
Original post by Mr M
Look at the coefficient of x.

Remember the sum of the roots is =ba=-\frac{b}{a}


what do you mean? b = 3 a = 1 so -b/a = -3?
Original post by Prokaryotic_crap
what do you mean? b = 3 a = 1 so -b/a = -3?


Do you remember doing this in A Level Further Maths?

I'm talking about the second equation (the one you are forming). Your sign is wrong for the coefficient of x.
Original post by Mr M
Do you remember doing this in A Level Further Maths?

I'm talking about the second equation (the one you are forming). Your sign is wrong for the coefficient of x.


yh, i remember?

so, 3x23x2=03x^2-3x-2=0?

I though the general form was ax2+bx+c=0ax^2+bx+c=0?

edit: Just spotted it!
(edited 11 years ago)

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