（1）-x²/4 + xy/3 - y²/9
= - (x²/4 - xy/3 + y²/9)
= - [(x/2)² - 2*(x/2)*(y/3) + (y/3)²]
= - (x/2 - y/3)²
(2) (x² + 1/4)² - 2x(x² + 1/4) + x²
= (x² + 1/4)² - 2*(x² + 1/4)*x + x²
= [(x² + 1/4) - x]²
= [x² - 2*x*(1/2) + (1/2)²]²
= [(x - 1/2)²]²
= (x - 1/2)^4