1.tìm x a,49*(x-21)=441 b,53*31*x=4929 c,69*(126:x)=1281 d,79*(x:23)=316 e,420:(x:39)=105 g,1615:(x*19)=17

Đáp án:

$\begin{array}{l}
a)49 \times \left( {x – 21} \right) = 441\\
 \Rightarrow x – 21 = 441:49\\
 \Rightarrow x – 21 = 9\\
 \Rightarrow x = 9 + 21\\
 \Rightarrow x = 30\\
b)\\
53 \times 31 \times x = 4929\\
 \Rightarrow 1643 \times x = 4929\\
 \Rightarrow x = 4929:1643\\
 \Rightarrow x = 3\\
c)\\
69 \times \left( {126:x} \right) = 1281\\
 \Rightarrow 126:x = 1281:69\\
 \Rightarrow 126:x = \frac{{427}}{{23}}\\
 \Rightarrow x = 126:\frac{{427}}{{23}}\\
 \Rightarrow x = \frac{{414}}{{61}}\\
d)\\
79 \times \left( {x:23} \right) = 316\\
 \Rightarrow x:23 = 316:79\\
 \Rightarrow x:23 = 4\\
 \Rightarrow x = 4 \times 23\\
 \Rightarrow x = 92\\
e)420:\left( {x:39} \right) = 105\\
 \Rightarrow x:39 = 420:105\\
 \Rightarrow x:39 = 4\\
 \Rightarrow x = 4 \times 39\\
 \Rightarrow x = 156\\
g)\\
1615:\left( {x \times 19} \right) = 17\\
 \Rightarrow x \times 19 = 1615:17\\
 \Rightarrow x \times 19 = 95\\
 \Rightarrow x = 95:19\\
 \Rightarrow x = 5
\end{array}$