Câu 39 : Trang 19 - sgk toán 8 tập 1
a) 3x - 6y;
b) \(\frac{2}{5}\)x2 + 5x3 + x2y;
c) 14x2y – 21xy2 + 28x2y2;
d) \(\frac{2}{5}\)x(y - 1) - \(\frac{2}{5}\)y(y - 1);
e) 10x(x - y) - 8y(y - x).
Bài Làm:
$a) 3x - 6y = 3 . x - 3 . 2y = 3(x - 2y)$
$b) \frac{2}{5}x^{2} + 5x^{3} + x^{2}y = x^{2} (\frac{2}{5} + 5x + y)$
$c) 14x^{2}y – 21xy^{2} + 28x^{2}y^{2} = 7xy . 2x - 7xy . 3y + 7xy . 4xy = 7xy(2x - 3y + 4xy)$
$d) \frac{2}{5}x(y - 1) - \frac{2}{5}y(y - 1) = \frac{2}{5}(y - 1)(x - y)$
$e) 10x(x - y) - 8y(y - x) = 10x(x - y) - 8y[-(x - y)]$
$= 10x(x - y) + 8y(x - y)$
$= 2(x - y)(5x + 4y)$