(3 x y - 16) x 7 = 84
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a, -(x+84) + 213=-16
-(x+84)+231=-16
-(x+84)=-16-231
-(x-84)=-247
=>x+84=247
x=247-84
x=163
b, -12 (x-15)+ 7(3-x) = 15
-12x+180+21-7x=15
(-12x-7x)+(180+21)=15
-19x+201=15
-19x=15-201
-19x=-186
x=-186:-19
x=186/19
\(\dfrac{x}{5}\) = \(\dfrac{48}{60}\)
\(x\) = \(\dfrac{48}{60}\) \(\times\) 5
\(x\) = 4
\(\dfrac{x}{7}\) = \(\dfrac{16}{56}\)
\(x\) = \(\dfrac{16}{56}\) \(\times\) 7
\(x\) = 2
\(\dfrac{8}{x}\) = \(\dfrac{72}{108}\)
\(x\) = 8 : \(\dfrac{72}{108}\)
\(x\) = 12
\(x\) - \(\dfrac{3}{12}\) = \(\dfrac{35}{84}\)
\(x\) = \(\dfrac{35}{84}\) + \(\dfrac{3}{12}\)
\(x\) = \(\dfrac{2}{3}\)
a) Ta có:+) \(\frac{12}{16}=\frac{-x}{4}\) <=> 12.4 = 16.(-x)
<=> 48 = -16x
<=> x = 48 : (-16) = -3
+) \(\frac{12}{16}=\frac{21}{y}\) <=> 12y = 21.16
<=> 12y = 336
<=> y = 336 : 12 = 28
+) \(\frac{12}{16}=\frac{z}{-80}\) <=> 12. (-80) = 16z
<=> -960 = 16z
<=> z = -960 : 16 = -60
b) Ta có: \(\frac{x+3}{7+y}=\frac{3}{7}\) <=> (x + 3).7 = 3(7 + y)
<=> 7x + 21 = 21 + 3y
<=> 7x = 3y
<=> \(\frac{x}{3}=\frac{y}{7}\)
Áp dụng t/c của dãy tỉ số bằng nhau, ta có:
\(\frac{x}{3}=\frac{y}{7}=\frac{x+y}{3+7}=\frac{20}{10}=2\)
=> \(\hept{\begin{cases}\frac{x}{3}=2\\\frac{y}{7}=2\end{cases}}\) => \(\hept{\begin{cases}x=2.3=6\\y=2.7=14\end{cases}}\)
Vậy ...
a) \(0,75\times26+49\times\frac{3}{4}+\frac{3}{4}\times24+\) \(0,75\)
\(=\) \(0,75\times\left(26+1\right)+\frac{3}{4}\times\left(49+24\right)\)
\(=\) \(\frac{3}{4}\times27+\frac{3}{4}\times73\)
\(=\) \(\frac{3}{4}\times\left(27+73\right)\)
\(=\)\(\frac{3}{4}\times100=75\)
b) câu b,c làm tương tự
+ Ta có:
\(\frac{x}{3}=\frac{y}{7}\) và \(x.y=84.\)
Đặt \(\frac{x}{3}=\frac{y}{7}=k.\)
\(\Rightarrow\left\{{}\begin{matrix}x=3k\\y=7k\end{matrix}\right.\)
+ Lại có: \(x.y=84\)
\(\Rightarrow3k.7k=84\)
\(\Rightarrow21.k^2=84\)
\(\Rightarrow k^2=84:21\)
\(\Rightarrow k^2=4\)
\(\Rightarrow k^2=\left(\pm2\right)^2\)
\(\Rightarrow k=\pm2.\)
+ TH1: \(k=2.\)
\(\Rightarrow\left\{{}\begin{matrix}x=3.2=6\\y=7.2=14\end{matrix}\right.\)
+ TH2: \(k=-2.\)
\(\Rightarrow\left\{{}\begin{matrix}x=3.\left(-2\right)=-6\\y=7.\left(-2\right)=-14\end{matrix}\right.\)
Vậy \(\left(x;y\right)=\left(6;14\right),\left(-6;-14\right).\)
Chúc bạn học tốt!
\(\left(3\cdot y-16\right)\cdot7=84\)
\(\Rightarrow3\cdot y-16=84:7\)
\(\Rightarrow3\cdot y-16=12\)
\(\Rightarrow3\cdot y=12+16\)
\(\Rightarrow3\cdot y=28\)
\(\Rightarrow y=\dfrac{28}{3}\)
\(\Leftrightarrow3y-16=12\)
\(\Leftrightarrow3y=28\)
\(\Leftrightarrow y=\dfrac{28}{3}\)