15. Làm tính nhân:
a) (\(\frac{1}{2}\)x + y)(\(\frac{1}{2}\)x + y); b) (x - \(\frac{1}{2}\)y)(x - \(\frac{1}{2}\)y)
a) (\(\frac{1}{2}\)x + y)(\(\frac{1}{2}\)x + y) = \(\frac{1}{2}\)x . \(\frac{1}{2}\)x + \(\frac{1}{2}\)x . y + y . \(\frac{1}{2}\)x + y . y
= \(\frac{1}{4}\)x2 + \(\frac{1}{2}\)xy + \(\frac{1}{2}\)xy + y2
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= \(\frac{1}{4}\)x2 + xy + y2
b) (x - \(\frac{1}{2}\)y)(x - \(\frac{1}{2}\)y) = x . x + x(-\(\frac{1}{2}\)y) + (-\(\frac{1}{2}\)y . x) + (-\(\frac{1}{2}\)y)(-\(\frac{1}{2}\)y)
= x2 - \(\frac{1}{2}\)xy - \(\frac{1}{2}\)xy + \(\frac{1}{4}\) y2
= x2 - xy + \(\frac{1}{4}\) y2