4x²+1/4-2x-9y²,1/4表示4分之1
=(4x²-2x+1/4)-9y²
=(2x-1/2)²-(3y)²
=(2x-1/2+3y)(2x-1/2-3y)
=(2x+3y-1/2)(2x-3y-1/2)