Bài 1: 1.1.Tìm x a, 3/35-(3/5+x)=2/7 b, 5x(2x-1/3)=0 c, 6(x-1)+2x(x-1)=0 1.2.Tính nhanh a, A=8/9-1/72-1/56-1/42-1/30-1/20-1/12-1/6-1/2 b, B=0,75-0,6+

0 bình luận về “Bài 1: 1.1.Tìm x a, 3/35-(3/5+x)=2/7 b, 5x(2x-1/3)=0 c, 6(x-1)+2x(x-1)=0 1.2.Tính nhanh a, A=8/9-1/72-1/56-1/42-1/30-1/20-1/12-1/6-1/2 b, B=0,75-0,6+”

1. Đáp án:

    $1.1
   a) x=\dfrac{-4}{5}\\
   b) {\left[\begin{aligned}x=0\\x=\dfrac{1}{6}\end{aligned}\right.}\\
   c)  {\left[\begin{aligned}x=1\\x=-3\end{aligned}\right.}\\
   1.2)
   a)
   A=0\\
   b)
   B=\dfrac{3}{11}$

   Giải thích các bước giải:

    $1.1
   a) \dfrac{3}{35}-\left ( \dfrac{3}{5}+x \right )=\dfrac{2}{7}\\
   \Leftrightarrow \dfrac{3}{35}-\dfrac{3}{5}-x=\dfrac{2}{7}\\
   \Leftrightarrow x=\dfrac{3}{35}-\dfrac{3}{5}-\dfrac{2}{7}\\
   \Leftrightarrow x=\dfrac{3}{35}-\dfrac{21}{35}-\dfrac{10}{35}\\
   \Leftrightarrow x=\dfrac{-28}{35}=\dfrac{-4}{5}$
   $b) 5x.\left ( 2x-\dfrac{1}{3} \right) =0\\
   \Leftrightarrow {\left[\begin{aligned}5x=0\\2x-\dfrac{1}{3}=0\end{aligned}\right.}\\
   \Leftrightarrow {\left[\begin{aligned}x=0\\2x=\dfrac{1}{3}\end{aligned}\right.}\\
   \Leftrightarrow {\left[\begin{aligned}x=0\\x=\dfrac{1}{6}\end{aligned}\right.}$
   $c) 6(x-1)+2x(x-1)=0\\
   \Leftrightarrow 2(x-1)(3+x)=0\\
   \Leftrightarrow {\left[\begin{aligned}x-1=0\\x+3=0\end{aligned}\right.}\\
   \Leftrightarrow {\left[\begin{aligned}x=1\\x=-3\end{aligned}\right.}$
   $1.2)
   a)
   A=\dfrac{8}{9}-\dfrac{1}{72}-\dfrac{1}{56}-\dfrac{1}{42}-\dfrac{1}{30}-\dfrac{1}{20}-\dfrac{1}{12}-\dfrac{1}{6}-\dfrac{1}{2}\\
   =-\left ( \dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72} \right )+\dfrac{8}{9}\\
   =-\left ( \dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9} \right )+\dfrac{8}{9}\\
   =-\left ( 1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9} \right )+\dfrac{8}{9}\\
   =-\left ( 1-\dfrac{1}{9} \right )+\dfrac{8}{9}\\
   =-1+\dfrac{1}{9}+\dfrac{8}{9}\\
   =-1+1\\
   =0$
   $b)
   B=\dfrac{0,75-0,6+\dfrac{3}{7}+\dfrac{3}{13}}{2,75-2,2+\dfrac{11}{7}+\dfrac{11}{13}}\\
   =\dfrac{\dfrac{3}{4}-\dfrac{3}{5}+\dfrac{3}{7}+\dfrac{3}{13}}{\dfrac{11}{4}-\dfrac{11}{5}+\dfrac{11}{7}+\dfrac{11}{13}}\\
   =\dfrac{3.\left (\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{7}+\dfrac{1}{13}  \right )}{11.\left (\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{7}+\dfrac{1}{13}  \right )}\\
   =\dfrac{3}{11}$

   Bình luận

2. Đáp án:

    

   Giải thích các bước giải:

    bài 1:

   `a) 3/35-(3/5+x)=2/7`

   `⇔3/5+x=3/35-2/7`

   `⇔3/5+x=-1/5`

   `⇔x=-1/5-3/5`

   `⇔x=-4/5`

   `b) 5x(2x-1/3)=0`

   `⇔x=0 hoặc 2x-1/3=0`

   `⇔x=0 hoặc x=1/6`

   `c) 6(x-1)+2x(x-1)=0`

   `⇔(x-1)(6+2x)=0`

   `⇔2(x-1)(3+x)=0`

   `⇔x-1=0 hoặc 3+x=0`

   `⇔x=1 hoặc x=-3`

   bài 2:

   `a) A=8/9-1/72-1/56-1/42-1/30-1/20-1/12-1/6-1/2`

   `A=8/9-(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72)`

   `A=8/9-(1/(1.2)+1/(2.3)+1/(3.4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8)+1/(8.9))`

   `A=8/9-(1-1/2+1/2-1/3+1/3-1/4+…+1/7-1/8+1/8-1/9)`

   `A=8/9-(1-1/9)`

   `A=8/9-8/9=0`

   `b) B=(0,75-0,6+3/7+3/13)/(2,75-2.2+11/7+11/13)`

   `B=(3/4-3/5+3/7+3/13)/(11/4-11/5+11/7+11/13)`

   `B=[3(1/4-1/5+1/7+1/13)]/[11(1/4-1/5+1/7+1/13)]`

   `B=3/11`

   Bình luận