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thì nó khó thiệt mà
\(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{a}{c}=\frac{b}{d}\Rightarrow\frac{a^2}{c^2}=\frac{b^2}{d^2}\Rightarrow\frac{2019a^2}{2019c^2}=\frac{2020b^2}{2020d^2}=\)
\(=\frac{2019a^2+2020b^2}{2019c^2+2020d^2}=\frac{2019a^2-2020b^2}{2019c^2-2020d^2}\Rightarrow\frac{2019a^2+2020b^2}{2019a^2-2020b^2}=\frac{2019c^2+2020d^2}{2019c^2-2020d^2}\)
Bạn ơi tham khảo thử cách này nhé !
Từ \(\frac{a}{b}=\frac{c}{d}\)( bài cho )
Đặt \(\frac{a}{b}=\frac{c}{d}=k\Rightarrow\hept{\begin{cases}a=bk\\c=dk\end{cases}}\)
Khi đó :
+) \(\frac{2019a^2+2020b^2}{2019a^2-2020b^2}=\frac{2019\left(bk\right)^2+2020b^2}{2019\left(bk\right)^2-2020b^2}=\frac{b^2\left(2019k^2+2020\right)}{b^2\left(2019k^2-2020\right)}=\frac{2019k^2+2020}{2019k^2-2020}\)
+) \(\frac{2019c^2+2020d^2}{2019c^2-2020d^2}=\frac{2019\left(dk\right)^2+2020d^2}{2019\left(dk\right)^2-2020d^2}=\frac{d^2\left(2019k^2+2020\right)}{d^2\left(2019k^2-2020\right)}=\frac{2019k^2+2020}{2019k^2-2020}\)
Ta có:
a/b = c/d => 2018a/2018b = 2018c/2018d = 2018a - 2018c / 2018b- 2018d
a/b = c/d => 2017a/2017b = 2017c/2017d =2017a+ 2017c/ 2017b+ 2017d
=> 2018a-2018c/2018b-2018d = 2017a+2017c/2017b+2017d (=a/b=c/d)
\(\dfrac{a}{b}=\dfrac{b}{c}=\dfrac{c}{d}=\dfrac{d}{e}=\dfrac{2019b+2020c-2021d}{2019c+2020d-2021e}\left(1\right)\\ \text{Đặt }\dfrac{a}{b}=\dfrac{b}{c}=k\Leftrightarrow a=bk;b=ck\Leftrightarrow a=ck^2\\ \Leftrightarrow\dfrac{a^2}{bc}=\dfrac{c^2k^4}{c^2k}=k^3=\left(\dfrac{a}{b}\right)^3\left(2\right)\\ \left(1\right)\left(2\right)\Leftrightarrow\left(\dfrac{2019b+2020c-2021d}{2019c+2020d-2021e}\right)^3=\dfrac{a^2}{bc}\)
Dăm ba mấy bài đặt k:v
Đặt \(\frac{a}{b}=\frac{c}{d}=k\)
Ta có:
\(\frac{2018a^2+2019b^2}{2018a^2-2019b^2}=\frac{2018b^2k^2+2019b^2}{2018b^2k^2-2019b^2}=\frac{b^2\left(2018k^2+2019\right)}{b^2\left(2018k^2-2019\right)}=\frac{2018k^2+2019}{2018k^2-2019}\)
\(\frac{2018c^2+2019d^2}{2018c^2-2019d^2}=\frac{2018d^2k^2+2019d^2}{2018d^2k^2-2019d^2}=\frac{d^2\left(2018k^2+2019\right)}{d^2\left(2018k^2-2019\right)}=\frac{2018k^2+2019}{2018k^2-2019}\)
Từ đó \(\frac{2018a^2+2019b^2}{2018a^2-2019b^2}=\frac{2018c^2+2019d^2}{2018c^2-2019d^2}\)
Từ \(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{a}{c}=\frac{b}{d}\)
\(\Rightarrow\frac{2018a}{2018c}=\frac{2019b}{2019d}\)
Áp dụng t/c DTSBN : \(\frac{2018a}{2018c}=\frac{2019b}{2019d}=\frac{2018a-2019b}{2018c-2019d}=\frac{2018a+2019b}{2018c+2019d}\)
Cái này đến đây là đề sai nhé ! Đề phải cho là C/m cái (2018a-2019b).(2018c+2019d) = (2018a-2019b)(2018c+2019d) mới đúng
Sửa đề: \(\dfrac{2018a-2019b}{2019a+2020b}=\dfrac{2018c-2019d}{2019c+2020d}\)
\(\dfrac{a}{b}=\dfrac{c}{d}\Leftrightarrow\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{2020a}{2020b}=\dfrac{2020c}{2020d}=\dfrac{2019a}{2019c}=\dfrac{2019b}{2019d}=\dfrac{2018a}{2018c}=\dfrac{2018b}{2018d}=\dfrac{2018a-2019b}{2018c-2019d}=\dfrac{2019a+2020b}{2019c+2020d}\\ \Leftrightarrow\dfrac{2018a-2019b}{2019a+2020b}=\dfrac{2018c-2019d}{2019c+2020d}\)
\(\dfrac{2018a-2019b}{2019c-2020d}=\dfrac{2018c-2018c}{2019a+2020b}\)
Sao .... ;-; ;-;