\(x\approx2,14\)

Cậu giải hộ tớ với

40,4 : (21-x)=x

40,4 : 21 -40,4:x=x 

\(40,4\div\left(21-x\right)=x.\)

\(\frac{404}{10}\div\left(21-x\right)=x.\)

\(\frac{2^2\times101}{2\times5}\div\left(21-x\right)=x..\)

\(\frac{2^{2-1}\times101}{5}\div\left(21-x\right)=x.\)

\(\frac{2\times101}{5}\div\left(21-x\right)=x.\)

\(\frac{202}{5}\times\frac{1}{21-x}=x\)

\(\frac{202}{5\left(21-x\right)}=x\)

\(202=5\left(21-x\right)x\)

\(202=5x\left(-x+21\right)\)

\(202=5x\left(-x\right)+5x\times21.\)

\(202=-5x^2+105x.\)

\(202+\left(5x^2-105x\right)=-5x^2+105x+5x^2-105x...\)

\(202+\left(5x^2-105x\right)=-5x^2+5x^2+105x-105x....\)

\(202+5x^2-105x=0.\)

\(5x^2-105x+202=0.\)

\(\hept{\begin{cases}x=\frac{-\left(-105\right)+\sqrt{105^2-4\times5\times202}}{2\times5}\\x=\frac{-\left(-105\right)-\sqrt{105^2-4\times5\times202}}{2\times5}\end{cases}}\)

\(\hept{\begin{cases}x=\frac{105+\sqrt{11025-4040}}{10}\\x=\frac{105-\sqrt{11025-4040}}{10}\end{cases}}\)

\(\hept{\begin{cases}x=\frac{105+\sqrt{6985}}{10}\\x=\frac{105-\sqrt{6985}}{10}\end{cases}}\)

\(x\in\left(\frac{105+\sqrt{6985}}{10},\frac{105-\sqrt{6985}}{10}\right).\)

\(x\in\left(18,85763...;2,14236...\right)\)

Học tốt nha, nhưng cái phép của tôi không phải ở lớp 5.

2,142368757 nhé bạn