Đáp án:
$3)
a) x=2\Rightarrow y=1\\
b)
y=\dfrac{-149}{6}\Rightarrow x=\dfrac{-23}{2}$
Giải thích các bước giải:
$3)
a) x+y=3\Rightarrow y=3-x\\
\dfrac{3x+4y}{5}=\dfrac{-2x+3y+9}{4}\\
\Leftrightarrow 4(3x+4y)=5.(-2x+3y+9)\\
\Leftrightarrow 12x+16y=-10x+15y+45\\
\Leftrightarrow 22x+y=45\\
\Rightarrow 22x+3-x=45\\
\Leftrightarrow 21x=42\\
\Leftrightarrow x=2\Rightarrow y=3-2=1\\
b)
7x-3y=-6\Rightarrow 7x=-6+3y\Rightarrow x=\dfrac{-6+3y}{7}\\
\dfrac{-3x+5y+3}{2}=\dfrac{5x+3y+2}{3}\\
\Leftrightarrow 3(-3x+5y+3)=2(5x+3y+2)\\
\Leftrightarrow -9x+15y+9=10x+6y+4\\
\Leftrightarrow 19x-9y=5\\
\Rightarrow 19.\dfrac{-6+3y}{7}-9y=5\\
\Leftrightarrow -114+57y-63y=35\\
\Leftrightarrow -6y=149\\
\Leftrightarrow y=\dfrac{-149}{6}\Rightarrow x=\dfrac{-6+3.\dfrac{-149}{6}}{7}=\dfrac{-23}{2}$
