7= 1. 7 = -1 . ( -7 )
ta co :
TH1: x- 1 = 1
x = 1+1
x = 2
y+ 7 = 7
y = 7 - 7
y = 0
TH2 : x-1 = 7
x = 7 + 1
x = 8
y + 7 = 1
y = 1 - 7
y = 1 + ( -7 )
y = -6
TH3 : x - 1 = -1
x = -1 +1
x = 0
y +7 = -7
y = -7 -7
y = -7 + ( -7 )
y = - 14
TH4; x- 1 = -7
x = -7 + 1
x = -6
y + 7 = -1
y = -1 - 7
y = -1 + ( -7 )
y = -8
vay ............
tick minh nha