HẰNG ĐẲNG THỨC HAY SỬ DỤNG (a + b) = a + 2ab + b = a − 2ab + b + 4ab = (a − b) + 4ab (a − b) = a − 2ab + b = a + 2ab + b − 4ab = (a + b) − 4ab a − b = (a − b)(a + b) (a + b)3 = a + 3a 2b + 3ab + b3 = a + b3 + 3ab(a + b) ⇒ a + b3 = (a + b)3 − 3ab(a + b) (a − b)3 = a − 3a 2b + 3ab − b3 = a − b3 − 3ab(a − b) ⇒ a − b3 = (a − b)3 + 3ab(a − b) a − b3 = (a − b)(a + ab + b ) a + b3 = (a + b)(a − ab + b ) (a + b + c)3 = a3 + b3 + c3 + 3( a + b)(b + c)(c + a) a + b3 + c − 3abc = (a + b + c )(a + b + c − ab − bc − ca ) 10 (a + b + c) = a + b + c + 2ab + 2bc + 2ca 11 (a − b + c) = a + b + c − 2ab − 2bc + 2ca 2 2 12 (a1 + a + a3 + + a n ) = a1 + a2 + + an + 2( a1a2 + a2 a3 + + an −1an ) 13 a + b5 = (a + b)(a − a 3b + a 2b − ab + b ) 14 a n + bn = (a + b)(a n −1 − a n − 2b + a n −3b − − ab n − + b n −1 ) 15 a − b5 = (a − b)(a + a 3b + a 2b + ab + b ) 16 a n − b n = (a − b)(a n −1 + a n− 2b + a n−3b + + ab n − + b n−1 ) ( với n lẻ ) 17 ( a + b2 ) = ( a + b ) + ( a − b ) 2 4 18 a − b = ( a + b ) ( a − b ) ( a + b ) − 2ab 19 a + b = (a + b )2 − 2a 2b = (a + b − ab 2)(a + b + ab 2) 2 2 2 20 a + a b + b = ( a + ab + b ) ( a − ab + b ) 2 21 a + a + = ( a + a + 1) ( a − a + 1) 22 ( a + b ) ( b + c ) ( c + a ) + abc = ( a + b + c ) ( ab + bc + ca )