3x" - 2x - 2 = 0
9x" - 6x - 6 = 0
( 9x" - 6x + 1 ) - 7 = 0
( 3x - 1 )" - (√7 )" = 0
( 3x - 1 - √7 )( 3x - 1 + √7 ) = 0
解方程得
x1 = ( 1 + √7 )/3
x2 = ( 1 - √7 )/3