\(a,21-22+23-24=-1+\left(-1\right)=-2\\ b,125-\left(115-99\right)=125-115+99=10+99=109\)

a) 21 - 22 + 23 - 24

= (21 + 23) + (-22 - 24)

= 44 - 46

= -2

b) 125 - (115 - 99)

= 125 - 115 + 99

= 10 + 99

= 109

a, - 2

b, 109

Câu 2:

a: x-158=32

=>x=158+32

=>x=190

b: \(x\cdot24=264\)

=>\(x=\dfrac{264}{24}\)

=>x=11

c: \(6x+9=3^7:3^4\)

=>\(6x+9=3^3\)

=>6x+9=27

=>6x=18

=>x=18/6=3

Câu 1:

a: \(86\cdot19+14\cdot19\)

\(=19\left(86+14\right)\)

\(=19\cdot100=1900\)

b: \(4\cdot\left(-5\right)^2-104\cdot\left(-5\right)^2\)

\(=4\cdot25-104\cdot25\)

\(=25\left(4-104\right)=-100\cdot25=-2500\)

c: \(7\cdot\left(-2\right)\cdot8\left(-5\right)\)

\(=7\cdot2\cdot8\cdot5\)

\(=56\cdot10=560\)

d: \(59-\left[59+\left(-76\right)\right]\)

\(=59-59+76\)

=76

a: \(A=\dfrac{1-\dfrac{1}{\sqrt{49}}+\dfrac{1}{49}-\dfrac{1}{\left(7\sqrt{7}\right)^2}}{\dfrac{\sqrt{64}}{2}-\dfrac{4}{7}+\left(\dfrac{2}{7}\right)^2-\dfrac{4}{343}}\)

\(=\dfrac{1-\dfrac{1}{7}+\dfrac{1}{49}-\dfrac{1}{343}}{4-\dfrac{4}{7}+\dfrac{4}{49}-\dfrac{4}{343}}\)

\(=\dfrac{1-\dfrac{1}{7}+\dfrac{1}{49}-\dfrac{1}{343}}{4\left(1-\dfrac{1}{7}+\dfrac{1}{49}-\dfrac{1}{343}\right)}=\dfrac{1}{4}\)

b: \(M=1-\dfrac{5}{\sqrt{196}}-\dfrac{5}{\left(2\sqrt{21}\right)^2}-\dfrac{\sqrt{25}}{204}-\dfrac{\left(\sqrt{5}\right)^2}{374}\)

\(=1-\dfrac{5}{14}-\dfrac{5}{84}-\dfrac{5}{204}-\dfrac{5}{374}\)

\(=1-5\left(\dfrac{1}{14}+\dfrac{1}{84}+\dfrac{1}{204}+\dfrac{1}{374}\right)\)

\(=1-5\left(\dfrac{1}{2\cdot7}+\dfrac{1}{7\cdot12}+\dfrac{1}{12\cdot17}+\dfrac{1}{17\cdot22}\right)\)

\(=1-\left(\dfrac{5}{2\cdot7}+\dfrac{5}{7\cdot12}+\dfrac{5}{12\cdot17}+\dfrac{5}{17\cdot22}\right)\)

\(=1-\left(\dfrac{1}{2}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{12}+\dfrac{1}{12}-\dfrac{1}{17}+\dfrac{1}{17}-\dfrac{1}{22}\right)\)

\(=1-\left(\dfrac{1}{2}-\dfrac{1}{22}\right)\)

\(=1-\dfrac{11-1}{22}=1-\dfrac{10}{22}=\dfrac{12}{22}=\dfrac{6}{11}\)

a; \(\dfrac{9}{4}\) - \(\dfrac{-11}{4}\)

= \(\dfrac{9}{4}\) + \(\dfrac{11}{4}\)

= \(\dfrac{20}{4}\)

= 5 

b; \(\dfrac{7}{8}\) - \(\dfrac{3}{-8}\) - \(\dfrac{1}{8}\)

=  \(\dfrac{7}{8}\) + \(\dfrac{3}{8}\) - \(\dfrac{1}{8}\)

= \(\dfrac{7+3-1}{8}\)

= \(\dfrac{9}{8}\) 

c; \(\dfrac{-5}{21}\) - \(\dfrac{25}{21}\) - \(\dfrac{-1}{21}\)

  = \(\dfrac{-5}{21}\) - \(\dfrac{25}{21}\) + \(\dfrac{1}{21}\)

 =  \(\dfrac{-5-25+1}{21}\)

= \(\dfrac{-29}{21}\)

1-2+3-4+5-6+7-8+9-10+11-12+13-14+15-16+17-18+19-20+21 = 11

1-2+3-4+.............+19-20+21

=(1-2)+(3-4)+...........+(19-20)+21

=(-1)+(-1)+............+(-1)+21{có 10 số -1)

=(-1).10+21

=(-10)+21

=11

Số đó là : 11

Đáp số : 11

6/21; 7/21; 8/21; 9/21; 10/21; 11/21; 12/21; 13/21; 14/21; 15/21; 16/21

Bạn cần làm gì với những phân số này?

Đề là j em nhỉ ?

tính

6/21; 7/21; 8/21; 9/21; 10/21; 11/21; 12/21; 13/21; 14/21; 15/21; 16/21

\(\dfrac{1}{21}+\dfrac{2}{21}+\dfrac{3}{21}+\dfrac{4}{21}+\dfrac{5}{21}+...+\dfrac{17}{21}+\dfrac{18}{21}+\dfrac{19}{21}+\dfrac{20}{21}\)

\(=\dfrac{1+2+3+4+5+...+17+18+19+20}{21}\)

\(=\dfrac{1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20}{21}\)

\(=\dfrac{210}{21}\)

\(=10\)

ai làm giúp mình vs ạ mình đang cần gấp