因式分解(2)

1) 50 - 8y^2

= 2 ( 25 - 4y^2 )

= 2 { 5^2 - ( 2y )^2 }

= 2 ( 5 + 2y )( 5 - 2y ) { 用恆等式: a^2 - b^2 = ( a + b )( a - b ) }

2) 20 - 7x - 6x^2

= - ( 6x^2 + 7x - 20 )

用十字相乘法,

2x + 5

3x - 4

-----------------

15x - 8x = 7x

所以,

20 - 7x - 6x^2

= - ( 3x - 4 )( 2x + 5 )

( 3 ) - 343 - 8y^2

= - ( 343 + 8y^2 )

= - { 7^3 + ( 2y )^3 }

= - ( 7 + 2y )( 49 - 14y + 4y^2 ) { 用恆等式: a^3 + b^3 = ( a + b )( a^2 - ab + b^2 ) }

4a) x^2 - 2xy + y^2

= ( x - y )^2

b) x^2 - 2x ( x + 1 ) + ( x + 1 )^2

= { x - ( x + 1 ) }^2 { 用恆等式: a^2 - 2ab + b^2 }

= ( x - x - 1 )^2

= ( - 1 )^2

= 1

１．　２（５－2y)(5+2y)

3. -7^3-2y^3
4a. (x-y)^2