For any x∈R, we have (x−b0)⋯(x−bn)−(x−a0)⋯(x−an)=c. Prove: n!∣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 x on both sides of the equation, we have ⎩⎨⎧b0+b1+⋯+bn=a0+a1+⋯+an,∑0⩽i⩽j⩽nbibj=∑0⩽i⩽j⩽naiaj,⋯⋯∑i=0nbib0⋯bn=∑i=0naia0⋯an,(−1)n+1(b0⋯bn−a0⋯an)=c,
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.
Consider the polynomial f(x)=(x−1)(x−2)⋯(x−(n+1))= set xn+1+d1xn+⋯+dn+1. i=0∑nf(bi)=i=0∑n(bi−1)(bi−2)⋯(bi−(n+1))=i=0∑n(bin+1+d1bin+⋯+dn+1)=(i=0∑nain+1)−(n+1)c+i=0∑n(d1ain+⋯+dn+1)=(i=0∑n(ai−1)(ai−2)⋯(ai−(n+1)))(n+1)c
Each term in the sums on both sides of the above equation is the product of n+1 consecutive integers, which is a multiple of (n+1)!. Therefore, (n+1)!∣i=0∑n(bi−1)⋯(bi−(n+1))−i=0∑n(ai−1)⋯(ai−(n+1)),
which implies (n+1)!∣(n+1)⇔n!∣c.
Source: NuminaMath-1.5,
licensed Apache-2.0.
Statement and solution reproduced as published; topic, difficulty and ordering added
by this site.