Hướng dẫn giải Bài 32 (Trang 16 SGK Toán Đại số 8, Tập 1)

Điền các đơn thức thích hợp vào ô trống: a) (3x + y)(<img id="equationview" title="This is the rendered form of the equation. You can not edit this directly. Right click will give you the option to save the image, and in most browsers you can drag the image onto your desktop or another program." src="http://img.loigiaihay.com/picture/article/2017/0119/bai-32-trang-16-sgk-toan-8-tap-1_1_1484822020.jpg" alt="" /> - <img id="equationview" title="This is the rendered form of the equation. You can not edit this directly. Right click will give you the option to save the image, and in most browsers you can drag the image onto your desktop or another program." src="http://img.loigiaihay.com/picture/article/2017/0119/bai-32-trang-16-sgk-toan-8-tap-1_1_1484822020.jpg" alt="" /> + <img id="equationview" title="This is the rendered form of the equation. You can not edit this directly. Right click will give you the option to save the image, and in most browsers you can drag the image onto your desktop or another program." src="http://img.loigiaihay.com/picture/article/2017/0119/bai-32-trang-16-sgk-toan-8-tap-1_1_1484822020.jpg" alt="" />) = 27x3 + y3 b) (2x - <img id="equationview" title="This is the rendered form of the equation. You can not edit this directly. Right click will give you the option to save the image, and in most browsers you can drag the image onto your desktop or another program." src="http://img.loigiaihay.com/picture/article/2017/0119/bai-32-trang-16-sgk-toan-8-tap-1_1_1484822020.jpg" alt="" />)(<img id="equationview" title="This is the rendered form of the equation. You can not edit this directly. Right click will give you the option to save the image, and in most browsers you can drag the image onto your desktop or another program." src="http://img.loigiaihay.com/picture/article/2017/0119/bai-32-trang-16-sgk-toan-8-tap-1_1_1484822020.jpg" alt="" /> + 10x + <img id="equationview" title="This is the rendered form of the equation. You can not edit this directly. Right click will give you the option to save the image, and in most browsers you can drag the image onto your desktop or another program." src="http://img.loigiaihay.com/picture/article/2017/0119/bai-32-trang-16-sgk-toan-8-tap-1_1_1484822020.jpg" alt="" />) = 8x3 - 125. Giải: a) Ta có: 27x3 + y3 = (3x)3 + y3                 = (3x + y)[(3x)2 – 3x . y + y2]                 = (3x + y)(9x2 – 3xy + y2) Nên: (3x + y) (9x2 – 3xy + y2 ) = 27x3 + y3 b) Ta có: 8x3 - 125 = (2x)3 - 53                 = (2x - 5)[(2x)2 + 2x . 5 + 52]                  = (2x - 5)(4x2 + 10x + 25) Nên: (2x - 5)(4x2+ 10x +25 ) = 8x3 - 125