RMO practice problems from past year papers (polynomials)
⎩⎪⎨⎪⎧P1(x)=ax2−bx−cP2(x)=bx2−cx−aP3(x)=cx2−ax−b
Let the above be three quadratic polynomials where a,b,c are non-zero real numbers. Suppose there exists a real number α such that P1(α)=P2(α)=P3(α), then
Prove that a=b=c.
Please post your answers after at least one week (although this one is quite simple).
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Make the coefficient of y2 same and subtract to eliminate it. For eg: multiply first equation by b and second by a and observe the coefficient of y2 and thereby subtract them.
Since, the three polynomials have same root. So, one of them can be expressed as linear combination of other two. So, P1(x)=P2(x)+λP3(x), where λ is a real number. Comparing the coefficients of x2,x and constant, we get different values of λ. They are
λ=bc−a=ca−b=ab−c=a+b+ca−b+b−c+c−a=0
which implies that
a=b=c
P(x)=4x3−2x2−15x+9 and Q(x)=12x3+6x2−7x+1 then prove that each polynomial has 3 distinct real roots and let a and b be greatest roots of p(x) and q(x), respectively then prove that a2+3b2=4
Easy Math Editor
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Comments
just put alpha at x. I am taking alpha as y.
ay2−by−c=k
by2−cy−a=k
cy2−ay−b=k
removing y2 from these equations, we get three other equations and add them all the we get
ab+bc+ca−a2−b2−c2=0
so a = b = c
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Nice!
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Thanks. Are you preparing for olympiad?
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How do you simply "remove" y2 from these equations?
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Make the coefficient of y2 same and subtract to eliminate it. For eg: multiply first equation by b and second by a and observe the coefficient of y2 and thereby subtract them.
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Simple standard solution.
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Change your Profile pic :3 xD
A good troll anyway :P
I got trolled xD
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Hey Nihar and Mehul, try this problem.
If x3=x+1 then determine integer a, b and c such that x7=ax2+bx+c
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x3=x+1
Squaring both sides yields
x6=(x+1)2=x2+2x+1
Since x is non zero we can multiply both sides by x and get
x7=x3+2x2+x
x7=x+1+2x2+x
x7=2x2+2x+1
So we get a=2,b=2
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Since, the three polynomials have same root. So, one of them can be expressed as linear combination of other two. So, P1(x)=P2(x)+λP3(x), where λ is a real number. Comparing the coefficients of x2,x and constant, we get different values of λ. They are
λ=bc−a=ca−b=ab−c=a+b+ca−b+b−c+c−a=0 which implies that a=b=c
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i did same.....
How do u know that the roots are equal?
@Dev Sharma @Swapnil Das @Nihar Mahajan @Mehul Arora @Shivam Jadhav
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Thanks for mentioning😛
Thanks for mentioning me! @Svatejas Shivakumar
@Silas Hundt There is some problem. I am not receiving notifications when someone @mentions me
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Which kind of notifications? The ones in the corner on Brilliant or emails or...?
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try this
P(x)=4x3−2x2−15x+9 and Q(x)=12x3+6x2−7x+1 then prove that each polynomial has 3 distinct real roots and let a and b be greatest roots of p(x) and q(x), respectively then prove that a2+3b2=4
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@Dev Sharma Can you post the solution?
I still did not understand how to solve the question
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Bt why
Thnx