( x" - 1 )( x" + 2 ) - 70
先用平方差算一算
= ( x" - 1 )( x" + 1 ) + ( x" - 1 ) - 70
= ( x" )" - 1 + x" - 1 - 70
= x^4 + x" - 72
分解因式
= ( x" )" + 9x" - 8x" - 72
= x"( x" + 9 ) - 8( x" + 9 )
= ( x" + 9 )( x" - 8 )