[Toán 8] Đại số 8

W

windysnow

$(5n - 2)^2 - (2n - 5)^2$

= $25n^2 - 20n + 4 - (4n^2 - 20n + 25)$

= $25n^2 - 20n + 4 - 4n^2 + 20n - 25$

= $25n^2 - 25 + 4 - 4n^2$

= $25(n^2 - 1) + 4(1 - n^2)$

= $25(n^2 - 1) - 4 (n^2 - 1)$

= $(n^2 - 1)(25 - 4)$

= $(n^2 - 1)21$ \Rightarrow đpcm

 
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