... a )2 - 6a = 9 - 6a + a2 - 6a = 9 - 12a + a2v¦i a <0, ta c¨ = (3 - a )2 -(- 6a) = 9 - 6a + a2 +6a = 9 -a2.ba`i 21 (sgk -tr 15) : khai ph¬ng t¦ch 12. 30.40 ®o+.c:(a) 120 0 (b) 120 (c) 12 (d) 24 0hu'y ... ta ®o+.c k¥t qua? sau:x -2, 5 -2, 25-1,5-1011,5 2, 25 2, 5y = 0,5x-1 ,25 -1, 125 -0,75-0,500,50,751, 125 1 ,25 y = 0,5x + 2 0,750,8751 ,25 1,5 2 2,5 2, 753, 125 3 ,25 b) khi bi¥n x la^'y ... thu+' ba):a) x2 = 2 x1 = va` x2 = - hay x1 va` x2 b) x2 = 3 x1 = va` x2 = - hay x1 va` x2 c) x2 = 3,5 x1 = va` x2 = - hay x1 va` x2 d) x2 = 4, 12 x1 = va` x2 = - hay x1 va` x2 .ba`i 4(sgk...