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Algebra Difficulty 5.9 AIME, harder Prove it Slovenia

Consider all polynomials with the leading coefficient 11 that give the remainder of 11 when divided by x+1x + 1 and the remainder 22 when divided by x2+1x^2 + 1. Among these, find the polynomial of the lowest degree.

Solution

Obviously, the degree of such a polynomial must be at least 22. If the degree were equal to 22, this polynomial would have the form of
p(x)=x2+ax+b=(x2+1)+ax+(b1). p(x) = x^2 + a x + b = (x^2 + 1) + a x + (b - 1).
When dividing by x2+1x^2 + 1 the remainder is 22, and ax+(b1)=2a x + (b - 1) = 2 implies a=0a = 0 and b=3b = 3. But when we divide p(x)=x2+3=(x1)(x+1)+4p(x) = x^2 + 3 = (x - 1)(x + 1) + 4 by x+1x + 1 the remainder cannot be 11. We can prove this by calculating p(1)=4p(-1) = 4, since p(1)p(-1) is precisely the remainder obtained when dividing p(x)p(x) by x+1x + 1.

We conclude that the degree of our polynomial is at least 33. Let
p(x)=x3+ax2+bx+c=(x+a)(x2+1)+(b1)x+ca. p(x) = x^3 + a x^2 + b x + c = (x + a)(x^2 + 1) + (b - 1)x + c - a.
Again, we have (b1)x+ca=2(b-1)x + c - a = 2. Hence b=1b = 1, c=a+2c = a + 2. From
p(x)=x3+ax2+x+a+2=(x+1)(x2+(a1)x+2a)+2a, p(x) = x^3 + a x^2 + x + a + 2 = (x + 1)(x^2 + (a - 1)x + 2 - a) + 2a,
we find 2a=12a = 1 (we can also deduce this from 1=p(1)=1a1+a+21 = p(-1) = -1 - a - 1 + a + 2). The polynomial is p(x)=x3+12x2+x+52p(x) = x^3 + \frac{1}{2}x^2 + x + \frac{5}{2}.

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