Toán Lớp 8: Bài 1 : Tính a) (3xy+5).2x b) (2x-3y)^2 c) (x+3)^2 -(x-2)(x+2) Bài 2 : Phân tích đa thức thành nhân tử a) 2xy^2-10xy b) x^2 +xy-5x-5y c

Question

Toán Lớp 8:  Bài 1 : Tính a) (3xy+5).2x b) (2x-3y)^2 c) (x+3)^2 -(x-2)(x+2) Bài 2 : Phân tích đa thức thành nhân tử a) 2xy^2-10xy b) x^2 +xy-5x-5y c) 25-x^2-y^2+2xy

thudan · Answer

1.   $a) (3xy+5).2x$   $=3xy.2x+5.2x$   $=6x^2y+10x$   $b) (2x-3y)^2$   $=(2x-3y)(2x-3y)$   $=4x^2-12xy+9y^2$   $c) (x+3)^2 -(x-2)(x+2)$   $= (x^2+6x+9) -(x^2-4)$   $= x^2+6x+9 -x^2+4$   $=6x+13$   2.   $a) 2xy^2-10xy$   $= 2xy.y-5.2xy$   $= 2xy.(y-5)$   $b) x^2 +xy-5x-5y$   $=(x.x +x.y)-(5x+5y)$   $=x(x +y)-5(x+y)$   $=(x-5)(x +y)$   $c) 25-x^2-y^2+2xy$   $=25-(x^2-2xy+y^2)$   $=5^2-(x-y)^2$   $=[5-(x-y)][5+(x-y)]$   $=(5-x+y)(5+x-y)$

dananhthumai · Answer

B&agrave;i 1 :   a) (3xy+5).2x   = 3xy.2x + 5.2x   = 6x^2y + 10x   b) (2x - 3y)^2   = (2x)^2 - 2.2x.3y + (3y)^2   = 4x^2 - 12xy + 9y^2   c) (x + 3)^2 - (x-2)(x+2)   = x^2 + 6x + 9 - (x^2 - 2^2)   = x^2 + 6x + 9 - x^2 + 2^2   = x^2 + 6x + 9 - x^2 + 4   = 6x + 13   B&agrave;i 2 :   a) 2xy^2 - 10xy   = 2xy(y-5)   b) x^2 + xy - 5x - 5y   = (x^2 + xy) - (5x+5y)   = x(x+y) - 5(x+y)   = (x+y)(x-5)   c) 25 - x^2 - y^2 + 2xy   = 5^2 - (x^2 + y^2 - 2xy)   = 5^2 - (x^2 - 2xy + y^2)   = 5^2 - (x-y)^2   = (5 - x + y)(5 + x - y)