# Help with a proof

given
a+bx+cx² = d+ex+fx², for all x, you know that a=d, b=e and c=f...
how does one go about proving this?
Any pointers would be appreciated

Philcorp

jcsd
Gold Member
Well the co-efficent of x^2 is given solely by a on the left and d on the right, for the two equations to be equal this co-efficent must be the same therfore a=d again the same with b and e for the co-efficent of x and also c and f must be equal as they give the units (or the co-efficent of x^0 if you like).

Hurkyl
Staff Emeritus
Gold Member
Form three equations by plugging in -1, 0, and 1 for x. Solve those equations for a, b, and c in terms of d, e, and f.

Hmmmm . . .
hmmm . . .

have you ever seen a proof such as if ax + by = cx + dy then a = c, and b = d for all x,y in the reals?

same principle

Originally posted by Hurkyl
Form three equations by plugging in -1, 0, and 1 for x. Solve those equations for a, b, and c in terms of d, e, and f.
How do you know that the numbers -1, 0, 1 will not give a special case result ?
i mean, i know if you plug in -1, 0, 1 you will get a=d, b=e, c=f, but how you would explain that this is not a special case (you know, you chose three special numbers, so the result can be special case result).

Hurkyl
Staff Emeritus
Gold Member
We're told that the equation holds for all x, so it must be true for any choices of x!

By solving the equations, we've proven that, if the equation is true for all x, then a = d, b = e, and c = f. It's trivial to go the other way and prove that if a = d, b = e, and c = f then the equation is true for all x.

suffian
A rather simple solution can be obtained by calculus:

a + bx + cx2 = d + ex + fx2
b + 2cx = e + 2fx (taking derivatives, think of each side as a function of x)
2c = 2f
c = f

Now plug in c = f, cancel out the x2 terms and repeat.

In this way, you could extend the applicability to a polynomial of any degree.

another(?) way is to just use the substitution x=0 again and again as follows:

if p(x)= ax^n + bx^(n-1)....+ z= 0 for all x

then @ x=0
p(0)=0 => z=0

then take x common and write
x(ax^(n-1) + bx^(n-2)....+ y) = 0
(y is the coefficient of x in p(x))

since this is of the form g(x)*h(x)=0 and g is not identically zero we have h(x)=0.
now substitute x=0 again and we get y=0.similarly all other coefficients turn out to be zero.

Sounds like it's not even necessary to prove. Aren't they equal by definition. If d = a, e = b, f = c

HallsofIvy
Homework Helper
No, they are not "equal by definition". The definition of
"f(x)= g(x)" is that for any value x= a, the values f(a) and g(a) are equal. While the value, of course, depends on the coefficients, it is incorrect to say that the definition of equality of two polynomials is that they have the same coefficients.

I would also note that jcsd's suggestion (which is essentially the same thing) works, it probably would not be accepted as a proof since I suspect that the purpose of this problem is to see WHY the fact that two polynomials have the same values for all x implies that they have the same coefficients.

In general

Lemma: If the polynomial f(x)=anxn+an-1xn-1...+a1x + a0 = 0 for more than n distinct values of x, then a0 = a0 =...=an = 0

Proof: Supose f(a1)=f(a2)=...=f(an+1)=0 where ai are distinct.

f(an+1) = an(an+1-a1)(an+1-a2)...(an+1-an) = 0
==> an=0
==>f(x)=an-1xn-1...+a1x + a0

Repeat the above process and the result follows.

Thm: Let
f(x) = anxn + an-1xn-1...+ a1x + a0
g(x) = bnxn + bn-1xn-1...+ b1x + b0
be 2 polynomials
If f(x) = g(x), then ai = bi for all i=1,2,3,...,n

Pf:
f(x)=g(x)
anxn + an-1xn-1...+ a1x + a0 = bnxn + bn-1xn-1...+ b1x + b0 "x belong to complex numbers (more than n distinct values of x)

(an-bn)xn + (an-1-bn-1)xn-1 + ... + (a0-b0) = 0
==> ai = bi

QED

Edit: typo

Last edited:
suffian

Originally posted by KL Kam
Lemma: If the polynomial f(x)=anxn+an-1xn-1...+a1x + a0 = 0 for more than n distinct values of x, then a0 = a0 =...=an = 0

Proof: Supose f(a1)=f(a2)=...=f(an+1)=0 where ai are distinct.

f(an+1) = an(an+1-a1)(an+1-a2)...(an+1-an) = 0
==> an=0
==>f(x)=an-1xn-1...+a1x + a0

The proof of the lemma seems a little sketchy to me; or, very confusing. It seems your using the same notation (ai) to refer to both the coeffecients and the roots of the polynomial.

Let me rewrite what I understand:

Suppose x1, x2, x3, ..., xn, xn+1 are roots of the polynomial.

Then you rewrite f(x) as follows:
f(x) = an(x - x1)(x - x2)...(x - xn)

My problem is this, how do you know you can rewrite f(x) as such (and knowing you can do so whether or not an != 0)?

Hurkyl
Staff Emeritus
Gold Member
Choose n to be the highest exponent of x with a nonzero coefficient.

f(x) can be factored as such because we know f(x) has n different roots, which means (x - xj) must be a factor of x for each root xj. Multiplying out all n of these factors makes it clear we must also have an additional factor an.

suffian
Originally posted by Hurkyl
Choose n to be the highest exponent of x with a nonzero coefficient.

f(x) can be factored as such because we know f(x) has n different roots, which means (x - xj) must be a factor of x for each root xj. Multiplying out all n of these factors makes it clear we must also have an additional factor an.

Okay, here is my point. You stated that if the highest coefficient is non-zero, then there are precisely n roots of f(x) (counting "double", "triple" roots their respective number of time). If we assume f(x) has n + 1 distinct roots, then we immediately know the highest coefficient must be zero for this situation to even possibly exist. Unfortunantly, because the highest coefficient is now zero, we don't know whether we can rewrite the polynomial as desired. Therefore, we must avoid taking that route in a proof.

Although from here the proof is relatively easy to complete. Now that the highest coefficient is zero we have a new equivalent polynomial of degree n-1 which equals f(x) (but we don't know whether this new polynomial's highest coefficient is zero). Again, since we have n+1 distinct roots to f(x), and our law states that a polynomial of degree n-1 can have at most n-1 distinct roots if it's highest coefficient is non-zero, then we know (since this is not the case) that the new equivalent polynomial of degree n-1 has a highest coefficient of zero. Repeating this for all coefficients completes the proof.

Hurkyl
Staff Emeritus
There are lots of ways to actually prove this. Though I think you and Kam have essentially the same proof, just Kam left out a detail or two. My mental proof is slightly different, and I would employ the "let m be the highest integer so that am is inequal to 0)" approach.
There are lots of ways to actually prove this. Though I think you and Kam have essentially the same proof, just Kam left out a detail or two. My mental proof is slightly different, and I would employ the "let m be the highest integer so that am is inequal to 0)" approach.