B=1/4.8/3.(-20).3/-8.-1/5

  =[1/4.-1/5.(-20)].[8/3.3/-8]

  =           1          .   (-1)

  =           -1

\(B=\left(0,25.\left(-20\right)\right).\left(\frac{8}{3}.\frac{3}{-8}\right).\frac{-7}{35}\)

\(B=-5\cdot\left(-1\right).\frac{-7}{35}\)

\(B=-1\)

3: 

\(=\dfrac{1}{7}\cdot\dfrac{3}{5}\cdot\dfrac{5}{6}\cdot\dfrac{5}{8}=\dfrac{1}{7}\cdot\dfrac{1}{2}\cdot\dfrac{5}{8}=\dfrac{5}{112}\)

4:

=>2/3:x=-2-1/3=-7/3

=>x=-2/3:7/3=-2/7

5:

AC=CB=12/2=6cm

IB=6/2=3cm

câu 1: tính lần lượt là dc

câu 2:

\(\frac{1}{2}+\frac{1}{6}+...+\frac{1}{2450}\)

\(=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{49.50}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{49}-\frac{1}{50}\)

\(=1-\frac{1}{50}\)

\(=\frac{49}{50}\)

a) =\(\left(\frac{1}{4}\right)^3\cdot2^5=\frac{1}{2^6}2^5=0,5\)

b) \(=\left(\frac{1}{8}\right)^3\cdot8^4\cdot10^4=80.000\)

c) \(=8^2\cdot\frac{4^5}{2^{20}}=\frac{2^6\cdot2^{10}}{2^{20}}=\frac{1}{2^4}=0,0625\)

d) = \(\frac{81^{11}\cdot3^{17}}{27^{10}\cdot9^{15}}=\frac{3^{44}\cdot3^{17}}{3^{30}\cdot3^{30}}=\frac{3^{61}}{3^{60}}=3\)

1: 0,35*12,4=0,35*2,4+0,35*10=3,5+0,84=4,34

2: =0,1-2,34=-2,24

3: =5-2,9=2,1

4: \(=2,5\left(10,124-0,124\right)=10\cdot2,5=25\)

5: =-3/7+1/13

=-39/91+7/91

=-32/91

6: =-1/3+1/3=0

\(A=\left(\frac{1}{3}-\frac{8}{15}-\frac{1}{7}\right)+\left[\frac{2}{3}-\left(-\frac{7}{15}\right)+\frac{8}{7}\right]\)

\(A=\frac{1}{3}-\frac{8}{15}-\frac{1}{7}+\frac{2}{3}+\frac{7}{15}+\frac{8}{7}\)

\(A=\left(\frac{1}{3}+\frac{2}{3}\right)+\left(-\frac{8}{15}+\frac{7}{15}\right)+\left(-\frac{1}{7}+\frac{8}{7}\right)\)

\(A=1+\left(-\frac{1}{15}\right)+1\)

\(A=\frac{29}{15}\)

\(B=\left[\left(0,25+\frac{3}{5}\right)-\left(\frac{1}{8}-\frac{2}{5}+\frac{5}{4}\right)\right]\)

\(B=\frac{1}{4}+\frac{3}{5}-\frac{1}{8}+\frac{2}{5}-\frac{5}{4}\)

\(B=\left(\frac{1}{4}-\frac{5}{4}\right)+\left(\frac{3}{5}+\frac{2}{5}\right)-\frac{1}{8}\)

\(B=-1+1-\frac{1}{8}\)

\(B=0-\frac{1}{8}\)
\(B=-\frac{1}{8}\)

\(=\frac{1}{3}-\frac{8}{15}-\frac{1}{7}+\frac{2}{3}+\frac{7}{15}+\frac{8}{7}\)

\(=\left(\frac{1}{3}+\frac{2}{3}\right)-\left(\frac{8}{15}+\frac{7}{15}\right)-\left(\frac{1}{7}-\frac{8}{7}\right)\)

\(=1-1+1\)

\(=0\)

a: \(B=\frac12-\left\lbrack\frac38+\left(-\frac74\right)\right\rbrack\)

\(=\frac12-\frac38+\frac74\)

\(=\frac48-\frac38+\frac{14}{8}=\frac{15}{8}\)

b: \(-\frac{4}{12}-\left(-\frac{13}{39}-0,25\right)+0,75\)

\(=-\frac13+\frac13+0,25+0,75\)

=0,25+0,75

=1

c: \(\frac12-\frac13+\frac{1}{23}+\frac16\)

\(=\left(\frac12-\frac13+\frac16\right)+\frac{1}{23}\)

\(=\left(\frac16+\frac16\right)+\frac{1}{23}=\frac13+\frac{1}{23}=\frac{26}{69}\)

d: \(\left(-\frac{13}{7}-\frac49\right)-\left(-\frac{10}{7}-\frac49\right)\)

\(=-\frac{13}{7}-\frac49+\frac{10}{7}+\frac49\)

\(=-\frac{13}{7}+\frac{10}{7}=-\frac37\)

e: \(\left(\frac78-\frac52+\frac47\right)-\left(-\frac37+1-\frac{13}{8}\right)\)

\(=\frac78-\frac52+\frac47+\frac37-1+\frac{13}{8}\)

\(=\frac{20}{8}-\frac52=0\)

f: \(-\frac37+\left(3-\frac34\right)-\left(2,25-\frac{10}{7}\right)\)

\(=-\frac37+3-\frac34-2,25+\frac{10}{7}\)

\(=\frac77+0,75-\frac34\)

=1
g: \(\left(\frac53-\frac37+9\right)-\left(2+\frac57-\frac23\right)+\left(\frac87-\frac43-10\right)\)

\(=\frac53-\frac37+9-2-\frac57+\frac23+\frac87-\frac43-10\)

\(=\left(\frac53+\frac23-\frac43\right)+\left(-\frac37-\frac57+\frac87\right)+\left(9-2-10\right)\)

\(=\frac33+7-10=1-3=-2\)