... ®æi: ( ) 2 13 2 324 32 2+=+=+ T¬ng tù:( ) 2 13 2 324 32 2−=−=− Do ®ã: ( ) ( )( ) ( ) ( ) ( )( )( )( )( ) 2 2 2 2 2 2 2 23 1 3 1 2 3 2 3 2 23 1 3 1 2 2 3 2 2 3 2 2 2 23 1 3 ... ta cã:( )() 2 22 2 22 2 2 2 2 2 2 2 2 122 122 111111111111xxxxxxxxxxxxxxA−−−=−−−−=−−+−=−−+−=−−−+=VËy -1 nÕu -1 < x < 0 A= 2 22 122 xxx−−− nÕu 0 ... chấm học sinh giỏi toán 9®Ò IIC©u 1: (1 ®iÓm) Ta cã : x + 2y = 1 <=> x =1 – 2y Do ®ã P = x.y = (1 - 2y)y = y – 2y 2 = -(2y 2 – y) 81 22 1 2 81 22 1 22 1 2 222 ≤−−=−−−yy...