Bài 1

1) 

a) 7-x=-2x+3

\(\Rightarrow-x+2x=3-7\\ \Rightarrow x=-4\)

\(\dfrac{y^2-14y-1}{y^2-4y+4}-y^2-6y\)

\(\Leftrightarrow\dfrac{y^2-14y-1}{y^2-4y+4}-\dfrac{\left(y^2+6y\right)\left(y^2-4y+4\right)}{y^2-4y+4}\)

\(\Rightarrow y^2-14y-1-\left(y^2+6y\right)\left(y^2-4y+4\right)\)

\(\Rightarrow\)y²-14y-1-(y⁴-4y³+4y²+6y³-24y²+24y)

\(\Rightarrow\)y²-14y-1-y⁴+4y³-4y²-6y³+24y²-24y

\(\Rightarrow\)-y⁴-2y³+21y²-38y-1

k)(a-c+b)²=a²+b²+c²+2ab-2bc-2ac

j)(a-b-c)²=a²+b²+c²-2ab+2bc-2ac

i)(a+b+c)²=a²+b²+c²+2ab+2bc+2ac

e)(y-2xy)²=y²-4xy²+4x²y²

d)\(\left(x^2+\dfrac{2}{5}y\right)\left(x^2-\dfrac{2}{5}y\right)\)=x⁴-\(\dfrac{4}{10}y^2\)

c)\(\left(\dfrac{x}{2}-\dfrac{yz}{6}\right)\left(\dfrac{x}{2}+\dfrac{yz}{6}\right)\)=\(\dfrac{x^2}{4}-\dfrac{\left(yz\right)^2}{36}\)