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@ @QPW@ @pý†bɾa@ @

21 ( ) 19 ( ) 17 ( ) 15 ( )

x () y x [AHSME 1968] ( )

−x ( ) y y =1+ 1 x =1+ 1
x y

1−x ( ) x −1 ()

66 [Mathcounts 1990] ( )

3 . 66

45 ( ) 41 ( ) 33 ( ) 21 ( )

x − 7 = 3− 7 [AHSME 1960] ( )
x −3 x −3 ()

()

() ( )

PQRS . 3 ABCD ( )

2 ABCD PQRS
ABCD
.4
5+3 5 ()
ABCD 5+5 3 ( )
9 [MAΘ 1991] ( )
5−3 5 ( )
5+3 3 ( )

16