Cách làm cho bạn:
a) 3(x – y)$^{2}$ - 2(x + y)$^{2}$ - (x – y)(x + y)
= 3(x$^{2}$ - 2xy + y$^{2}$) – 2(x$^{2}$ + 2xy + y$^{2}$) – (x$^{2}$ - y$^{2}$)
= 3x$^{2}$ - 2x$^{2}$ - x$^{2}$ + 3y$^{2}$ - 2y$^{2}$ + y$^{2}$ - 6xy – 4xy = 2y$^{2}$ - 10xy.
b) 2(2x + 5)$^{2}$ - 3(4x + 1)(1 – 4x)
= 2(4x$^{2}$ + 20x + 25) + 3(16x$^{2}$ - 1)
= 8x$^{2}$ + 40x + 50 + 48x$^{2}$ - 3 = 56x$^{2}$ + 40x + 47.
c) (x – 4)$^{2}$ - 2(x – 4)(x + 5) + (x + 5)$^{2}$
= [(x – 4) – (x – 5)]$^{2}$ = 1$^{2}$ = 1.
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