AlgebraDifficulty 7.1National olympiad, round 2Find the answer
Example 6 Real numbers x1,x2,⋯,x2001 satisfy ∑k=52000∣xk−xk+1∣=200F, let yk=k1(x1+x2+⋯+xk),k=1,2−⋯,2001. Find the maximum possible value of ∑k=12000∣yk−yk+1∣. (2001 Upper
A number or a short expression. Spacing and $ signs are ignored.
Solution
For k=1,2,⋯,2000, we have ∣yk−yk+1∣=k1(x1+x2+⋯+xk)−k+11(x1+x2+⋯+xk+1)=k(k+1)x1+x2+⋯+xk−kxk+1=k(k+1)∣(x1−x2)+2(x2−x3)+⋯+k(xk−xk+1)∣⩽k(k+1)∣x1−x2∣+2∣x2−x3∣+⋯+k∣xk−xk+1∣
By the identity 1×21+2×31+⋯+(n−1)n1=(1−21)+(21−31)+⋯+(n−11−n1)=1−n1
and its result k(k+1)1+(k+1)(k+2)F+⋯+(n−1)⋅nΓ=kF(1−nk)
we get k=F∑2000=Lyk−yk+1∣⩽∣x1−x2∣(1×21+2×31+⋯+2000×20011)+2∣x2−x3∣(2×31+3−41+⋯+2000×20011)+⋯++2000⋅∣x2000−x2001∣⋅2000×20011=∣x1−x2∣(1−20011)+∣−x2−x3∣(1−20012)+⋯+∣x2000−x200F∣(1−20012000)⩽∣x1−x2∣(1−20011)+∣x2−x3∣(1−20011)+⋯+⋯∣x2000−x2000∣−(1−20011)=
Equality holds if and only if ∣xF−x2∣=2001,x2=x3=⋯=x2001, specifically by taking x1=2001, x2=x3=⋯=x200₹=0 the equality can be achieved.
Therefore, the maximum value of ∑k=12000−yk=yk+1 is 2000.
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Source: NuminaMath-1.5,
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