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China Team Selection Test
2024 China Team Selection Test
18
Finally an Inequality
Finally an Inequality
Source: 2024 CTST P18
March 25, 2024
2024 CTST
inequalities
conbinatorics
Problem Statement
Let
m
,
n
∈
Z
≥
0
,
m,n\in\mathbb Z_{\ge 0},
m
,
n
∈
Z
≥
0
,
a
0
,
a
1
,
…
,
a
m
,
b
0
,
b
1
,
…
,
b
n
∈
R
≥
0
a_0,a_1,\ldots ,a_m,b_0,b_1,\ldots ,b_n\in\mathbb R_{\ge 0}
a
0
,
a
1
,
…
,
a
m
,
b
0
,
b
1
,
…
,
b
n
∈
R
≥
0
For any integer
0
≤
k
≤
m
+
n
,
0\le k\le m+n,
0
≤
k
≤
m
+
n
,
define
c
k
:
=
max
i
+
j
=
k
a
i
b
j
.
c_k:=\max_{i+j=k}a_ib_j.
c
k
:=
max
i
+
j
=
k
a
i
b
j
.
Proof
1
m
+
n
+
1
∑
k
=
0
m
+
n
c
k
≥
1
(
m
+
1
)
(
n
+
1
)
∑
i
=
0
m
a
i
∑
j
=
0
n
b
j
.
\frac 1{m+n+1}\sum_{k=0}^{m+n}c_k\ge\frac 1{(m+1)(n+1)}\sum_{i=0}^{m}a_i\sum_{j=0}^{n}b_j.
m
+
n
+
1
1
k
=
0
∑
m
+
n
c
k
≥
(
m
+
1
)
(
n
+
1
)
1
i
=
0
∑
m
a
i
j
=
0
∑
n
b
j
.
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