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Bài 1:
a) (2x+5)(x-6)=2x2+5x-12x-30=2x2-7x-30
b) (2x-1)(x2-4x+3)=2x3-8x2+6x-x2+4x-3=2x3-9x2+10x-3
c) x2-2x-(x-7)(x+2)=x2-2x-x2+7x-2x+14=3x+14
d) 3x-(x+2)(x+4)=3x-x2-2x-4x-8=-x2-3x-8
Bài 2:
a) 2(x+1)=x-1
⇒2x+2=x-1
⇒2x+2-x+1=0
⇒x+3=0
⇒x=-3
b) x(x+2)-x2=1
⇒x2+2x-x2=1
⇒2x=1
⇒x=0,5
c) 3x(x-2)=(3x-1)(x-1)-5
⇒3x2-6x=3x2-x-3x+1-5
⇒3x2-6x-3x2+x+3x-1+5=0
⇒-2x+4=0
⇒-2x=-4
⇒x=2
d) 6(x-1)(x-2)-6x(x+3)=2x
⇒6(x2-x-2x+2)-6x2-18x-2x=0
⇒6x2-6x-12x+12-6x2-18x-2x=0
⇒-38x+12=0
⇒-38x=-12
⇒x=\(\dfrac{6}{19}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
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![](https://rs.olm.vn/images/avt/0.png?1311)
\(IM=\dfrac{1}{4}IB\Rightarrow IM=\dfrac{1}{5}BM\Rightarrow\overrightarrow{MI}=\dfrac{1}{5}\overrightarrow{MB}=-\dfrac{1}{10}\left(\overrightarrow{BC}+\overrightarrow{BD}\right)\)
\(\Rightarrow\overrightarrow{DI}=\overrightarrow{DM}+\overrightarrow{MI}=\dfrac{1}{2}\overrightarrow{DC}-\dfrac{1}{10}\left(\overrightarrow{BC}+\overrightarrow{BD}\right)=\dfrac{1}{2}\overrightarrow{DB}+\dfrac{1}{2}\overrightarrow{BC}-\dfrac{1}{10}\overrightarrow{BC}-\dfrac{1}{10}\overrightarrow{BD}\)
\(\Rightarrow\overrightarrow{DI}=\dfrac{2}{5}\overrightarrow{BC}-\dfrac{3}{5}\overrightarrow{BD}\)
\(\overrightarrow{DJ}=\overrightarrow{DC}+\overrightarrow{CJ}=\overrightarrow{DB}+\overrightarrow{BC}+x.\overrightarrow{CB}=\left(1-x\right)\overrightarrow{BC}-\overrightarrow{BD}\)
D; I; J thẳng hàng \(\Rightarrow\dfrac{1-x}{\dfrac{2}{5}}=\dfrac{1}{\dfrac{3}{5}}\Rightarrow x=\dfrac{1}{3}\)
\(\Rightarrow CJ=\dfrac{1}{3}CB\Rightarrow BJ=\dfrac{2}{3}BC\Rightarrow\dfrac{BJ}{BC}=\dfrac{2}{3}\)
Gọi N là trung điểm AD \(\Rightarrow\dfrac{BG}{BN}=\dfrac{2}{3}\) (theo t/c trọng tâm)
\(\Rightarrow\dfrac{BJ}{BC}=\dfrac{BG}{BN}\Rightarrow JG||CN\)
\(\Rightarrow\widehat{\left(JG;CD\right)}=\widehat{\left(CN;CD\right)}=\widehat{NCD}=30^0\) (do tam giác ACD đều)
![](https://rs.olm.vn/images/avt/0.png?1311)
Tác dụng thì mở sách ra nha bn, còn cách lm thì phải tự thân vận động thôi
100
?? = 150