MathDB
Problems
Contests
National and Regional Contests
Ukraine Contests
Official Ukraine Selection Cycle
Ukraine Team Selection Test
2011 Ukraine Team Selection Test
4
4
Part of
2011 Ukraine Team Selection Test
Problems
(1)
a_i+ a_i* =a _ {i-1} +a_ {i + 1}
Source: Ukraine TST 2011 p4
5/7/2020
Suppose an ordered set of
(
a
1
,
a
2
,
…
,
a
n
)
({{a} _{1}}, \ {{a} _{2}},\ \ldots,\ {{a} _{n}})
(
a
1
,
a
2
,
…
,
a
n
)
real numbers,
n
≥
3
n \ge 3
n
≥
3
. It is possible to replace the number
a
i
{{a} _ {i}}
a
i
,
i
=
2
,
n
−
1
‾
i = \overline {2, \ n-1}
i
=
2
,
n
−
1
by the number
a
i
∗
a_ {i} ^ {*}
a
i
∗
that
a
i
+
a
i
∗
=
a
i
−
1
+
a
i
+
1
{{a} _ {i}} + a_ {i} ^ {*} = {{a} _ {i-1}} + {{a} _ {i + 1}}
a
i
+
a
i
∗
=
a
i
−
1
+
a
i
+
1
. Let
(
b
1
,
b
2
,
…
,
b
n
)
({{b} _ {1}},\ {{b} _ {2}}, \ \ldots, \ {{b} _ {n}})
(
b
1
,
b
2
,
…
,
b
n
)
be the set with the largest sum of numbers that can be obtained from this, and
(
c
1
,
c
2
,
…
,
c
n
)
({{c} _ {1}},\ {{c} _ {2}}, \ \ldots, \ {{c} _ {n}})
(
c
1
,
c
2
,
…
,
c
n
)
is a similar set with the least amount. For the odd
n
≥
3
n \ge 3
n
≥
3
and set
(
1
,
3
,
…
,
n
,
2
,
4
,
…
,
n
−
1
)
(1,\ 3, \ \ldots, \ n, \ 2, \ 4, \ \ldots,\ n-1)
(
1
,
3
,
…
,
n
,
2
,
4
,
…
,
n
−
1
)
find the values of the expressions
b
1
+
b
2
+
…
+
b
n
{{b} _ {1}} + {{b} _ {2}} + \ldots + {{b} _ {n}}
b
1
+
b
2
+
…
+
b
n
and
c
1
+
c
2
+
…
+
c
n
{{c} _ {1}} + {{c} _ {2}} + \ldots + {{c} _ {n}}
c
1
+
c
2
+
…
+
c
n
.
combinatorics
Sum
Sets