Maths Olympiad Prep

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Algebra Difficulty 4.8 AIME Find the answer Canada

Suppose that f(x)=x4+px3+qx2+rx+sf(x)=x^4+px^3+qx^2+rx+s for some real
numbers pp, qq, rr, ss. In addition, f(1)=59f(1)=59, f(2)=118f(2)=118 and f(3)=177f(3)=177. If T=f(9)+f(5)T=f(9)+f(-5), what is the sum of the
digits of the integer equal to TT?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Consider the function g(x)=59xg(x)=59x. Observe that g(1)=59g(1)=59, g(2)=118g(2)=118, and g(3)=177g(3)=177, from which it follows that
f(1)=g(1)f(1)=g(1), f(2)=g(2)f(2)=g(2), and f(3)=g(3)f(3)=g(3).

Now define a polynomial h(x)=f(x)g(x)h(x)=f(x)-g(x) so that, by construction,
x=1x=1, x=2x=2, and x=3x=3 are roots of h(x)h(x).

Algebraically, we also have that h(x)=f(x)g(x)=x4+px3+qx2+rx+s59x=x4+px3+qx2+(r59)x+sh(x) = f(x)-g(x) = x^4+px^3+qx^2+rx+s - 59x = x^4 + px^3 + qx^2 + (r-59)x+s but the important thing to notice here is that h(x)h(x) is a degree four polynomial with a
leading coefficient of 11.

Since we already know that x=1x=1,
x=2x=2, and x=3x=3 are roots of h(x)h(x), we conclude that h(x)=(x1)(x2)(x3)k(x)h(x)=(x-1)(x-2)(x-3)k(x) for some
polynomial k(x)k(x). However, because
h(x)h(x) has degree 44 and a leading coefficient of 11, it must be true that k(x)k(x) has degree 11 and a leading coefficient of 11.

Thus, h(x)=(x1)(x2)(x3)(xa)h(x) = (x-1)(x-2)(x-3)(x-a)
for some real number aa.

We now evaluate h(x)h(x) at x=9x=9 and x=5x=-5 to get h(9)=(91)(92)(93)(9a)=336(9a)h(5)=(51)(52)(53)(5a)=336(5+a)\begin{align*} h(9) &= (9-1)(9-2)(9-3)(9-a) = 336(9-a) \\ h(-5) &= (-5-1)(-5-2)(-5-3)(-5-a) = 336(5+a)\end{align*}
Now using that h(x)=f(x)g(x)h(x) = f(x)-g(x) or
f(x)=h(x)+g(x)f(x) = h(x)+g(x), we can determine
the value of f(9)+f(5)f(9)+f(-5) as follows:
f(9)+f(5)=h(9)+g(9)+h(5)+g(5)=336(9a)+9(59)+336(5+a)5(59)=336(9)336a+9(59)+336(5)+336a5(59)=336(9+5)+59(95)=336(14)+59(4)=4940\begin{align*} f(9) + f(-5) &= h(9)+g(9) + h(-5)+g(-5) \\ &= 336(9-a) + 9(59) + 336(5+a) -5(59) \\ &= 336(9)-336a+9(59)+336(5)+336a-5(59) \\ &= 336(9+5)+59(9-5) \\ &= 336(14)+59(4) \\ &= 4940\end{align*} Therefore, T=4940T=4940, so the answer is 4+9+4+0=174+9+4+0=17.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.