Olympiad Maths Prep

Track / Stage 6 / 160 of 400 #1160 of 2000

Problem 1160

National olympiad, first round
Algebra Difficulty 6.2 Prove it

For any xRx \in \mathbf{R}, we have (xb0)(xbn)(xa0)(xan)=c\left(x-b_{0}\right) \cdots\left(x-b_{n}\right)-\left(x-a_{0}\right) \cdots\left(x-a_{n}\right)=c.
Prove: n!cn! \mid c.

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

By comparing the coefficients of each power of xx on both sides of the equation, we have
{b0+b1++bn=a0+a1++an,0ijnbibj=0ijnaiaj,i=0nb0bnbi=i=0na0anai,(1)n+1(b0bna0an)=c, \left\{\begin{array}{l} b_{0}+b_{1}+\cdots+b_{n}=a_{0}+a_{1}+\cdots+a_{n}, \\ \sum_{0 \leqslant i \leqslant j \leqslant n} b_{i} b_{j}=\sum_{0 \leqslant i \leqslant j \leqslant n} a_{i} a_{j}, \\ \cdots \cdots \\ \sum_{i=0}^{n} \frac{b_{0} \cdots b_{n}}{b_{i}}=\sum_{i=0}^{n} \frac{a_{0} \cdots a_{n}}{a_{i}}, \\ (-1)^{n+1}\left(b_{0} \cdots b_{n}-a_{0} \cdots a_{n}\right)=c, \end{array}\right.

Thus, by Newton's formulas, we have
{b0+b1++bn=a0+a1++an,b02+b12+bn2=a02+a12++an2,b0n+b1n+bnn=a0n+a1n++ann,b0n+1+b1n+1++bnn+1=a0n+1+a1n+1++ann+1(n+1)c. \left\{\begin{array}{l} b_{0}+b_{1}+\cdots+b_{n}=a_{0}+a_{1}+\cdots+a_{n}, \\ b_{0}^{2}+b_{1}^{2}+\cdots b_{n}^{2}=a_{0}^{2}+a_{1}^{2}+\cdots+a_{n}^{2}, \\ \cdots \cdots \\ b_{0}^{n}+b_{1}^{n}+\cdots b_{n}^{n}=a_{0}^{n}+a_{1}^{n}+\cdots+a_{n}^{n}, \\ b_{0}^{n+1}+b_{1}^{n+1}+\cdots+b_{n}^{n+1}=a_{0}^{n+1}+a_{1}^{n+1}+\cdots+a_{n}^{n+1}-(n+1) c . \end{array}\right.

Consider the polynomial f(x)=(x1)(x2)(x(n+1))= set xn+1+d1xn++dn+1f(x)=(x-1)(x-2) \cdots(x-(n+1)) \stackrel{\text { set }}{=} x^{n+1}+d_{1} x^{n}+\cdots+d_{n+1}.
i=0nf(bi)=i=0n(bi1)(bi2)(bi(n+1))=i=0n(bin+1+d1bin++dn+1)=(i=0nain+1)(n+1)c+i=0n(d1ain++dn+1)=(i=0n(ai1)(ai2)(ai(n+1)))(n+1)c \begin{aligned} \sum_{i=0}^{n} f\left(b_{i}\right) & =\sum_{i=0}^{n}\left(b_{i}-1\right)\left(b_{i}-2\right) \cdots\left(b_{i}-(n+1)\right)=\sum_{i=0}^{n}\left(b_{i}^{n+1}+d_{1} b_{i}^{n}+\cdots+d_{n+1}\right) \\ & =\left(\sum_{i=0}^{n} a_{i}^{n+1}\right)-(n+1) c+\sum_{i=0}^{n}\left(d_{1} a_{i}^{n}+\cdots+d_{n+1}\right) \\ & =\left(\sum_{i=0}^{n}\left(a_{i}-1\right)\left(a_{i}-2\right) \cdots\left(a_{i}-(n+1)\right)\right)(n+1) c \end{aligned}

Each term in the sums on both sides of the above equation is the product of n+1n+1 consecutive integers, which is a multiple of (n+1)!(n+1)!. Therefore,
(n+1)!i=0n(bi1)(bi(n+1))i=0n(ai1)(ai(n+1)), (n+1)!\mid \sum_{i=0}^{n}\left(b_{i}-1\right) \cdots\left(b_{i}-(n+1)\right)-\sum_{i=0}^{n}\left(a_{i}-1\right) \cdots\left(a_{i}-(n+1)\right),

which implies (n+1)!(n+1)n!c(n+1)!|(n+1) \Leftrightarrow n!| c.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.