Hằng đẳng thức ` a^2 - b^2 = (a-b)(a+b)`
Bài làm
`a)`
` (2x+y)^2 -1 = (2x+y)^2 -1^2`
` = ( 2x+y-1)(2x+y+1)`
`b)`
` (x+y+z)^2 - (x-y-z)^2 `
` = [ (x+y+z) - (x-y-z)] . [ (x+y+z) + (x-y-z)]`
` = ( x + y +z - x + y + z)( x + y +z +x - y - z)`
` = ( 2y + 2z) . 2x`
` = 2. (y+z) . 2x`
` = 4x(y+z)`