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0<a <b <c <d [AHSME 1996] ( )
b +d ( ) b +c ( ) c +d ( ) a +b ()
a +c a +d a +b c +d
n −1 n +1
) [Aust.Math.Comp.1981] ( )
(
n +4 n−5 n−6
n−4 ( ) n−5 ( ) n −1 ( ) n +1 ()
11 7 1 [Aust.Math.Comp.1983] ( )
2 )
x . xx
5() )
4( ) 3( ) 2 () )
)
200 ≤ a ≤ 400 [AJHSME 1986] (
b 600 ≤ b ≤ 1200
a
600 ( ) 6( ) 3 ()
300 ( ) 2
[AJHSME 1987] (
4() 17 ( ) 100 ( ) 151 ( )
9 35 201 301
11
( ) 11 : 122 (
108 512 224 ( ) 12 ( )
12 ( ) 12
[AMC8 2001] (
