Question

số các bộ 3 số tự nhiên thỏa mãn 1/a+1/b+1/c=4/5

Answer 1

\(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=\frac{4}{5}\\ =>5\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)=5.\frac{4}{5}.\\ =>\frac{5}{a}+\frac{5}{b}+\frac{5}{c}=4\\ \)

\(\frac{5bc}{abc}+\frac{5ac}{abc}+\frac{5ab}{abc}=4\\ =>\frac{5bc+5ac+5ab}{abc}=4\)

\(=>1500+bc+ac+ab=4.abc\\ =>1500+20a+11b+2c=4.abc\)

Xin lỗi tới đây mình hàng 

Answer 2

 

$\ge b\ge c>0$

$\Rightarrow \frac{1}{c}\ge \frac{1}{b}\ge \frac{1}{a}$

$\Rightarrow \frac{1}{c}+\frac{1}{b}+\frac{1}{a}\le \frac{1}{c}+\frac{1}{c}+\frac{1}{c}$

$\iff \frac{4}{5}\le \frac{3}{c}$

$\Rightarrow 4c\le 15$

$\iff c\le 15:4$

$\iff c\le 3,75$

Mà $c\in N\ast \Rightarrow c\in \{1;2;3\}.$

Ta có $3$ trường hợp:

$T{}^{}:$

$c=1$ $\Rightarrow \frac{}{}$