( 1/25 )x" - ( 0.16 )y"= ( 1/25 )x" - ( 4/25 )y"= ( 1/25 )( x" - 4y" )= ( 1/25 )( x - 2y )( x + 2y )
平方差原式=(1/5x+0.4y)(1/5x-0.4y)
