9 : 2 + 392 : 8 = 24

Lời giải:

a.

$\frac{5}{15}-\frac{1}{6}\times \frac{2}{5}=\frac{5}{15}-\frac{1}{15}=\frac{4}{15}$

b.

$\frac{8}{24}+\frac{3}{4}:\frac{1}{8}=\frac{1}{3}+6=\frac{19}{3}$

c.

$\frac{1}{7}: \frac{2}{8}-\frac{1}{7}=\frac{1}{7}\times 4-\frac{1}{7}$

$=\frac{1}{7}\times (4-1)=\frac{1}{7}\times 3=\frac{3}{7}$

Câu 2.Tính giá trị biểu thức: 

8 giờ - (2 giờ 24 phút + 1 giờ 55 phút)

= 8 giờ - 3 giờ 79 phút

= 7 giờ 60 phút - 4 giờ 19 phút

= 3 giờ 41 phút

Ta có:

\(\begin{array}{l}\sin \frac{\pi }{{24}}\cos \frac{{5\pi }}{{24}} = \frac{1}{2}\left[ {\sin \left( {\frac{\pi }{{24}} + \frac{{5\pi }}{{24}}} \right) + \sin \left( {\frac{\pi }{{24}} - \frac{{5\pi }}{{24}}} \right)} \right]\\ = \frac{1}{2}\left[ {\sin \left( {\frac{\pi }{4}} \right) + \sin \left( { - \frac{\pi }{6}} \right)} \right]\\ = \frac{1}{2}\left[ {\frac{{\sqrt 2 }}{2} - \frac{1}{2}} \right] = \frac{{\sqrt 2  - 1}}{4}\end{array}\)

Ta có:

\(\begin{array}{l}\sin \frac{{7\pi }}{8}\sin \frac{{5\pi }}{8} = \frac{1}{2}\left[ {\cos \left( {\frac{{7\pi }}{8} - \frac{{5\pi }}{8}} \right) - \cos \left( {\frac{{7\pi }}{8} + \frac{{5\pi }}{8}} \right)} \right]\\ = \frac{1}{2}\left[ {\cos \left( {\frac{\pi }{4}} \right) - \cos \left( {\frac{{3\pi }}{2}} \right)} \right]\\ = \frac{1}{2}.\left( {\frac{{\sqrt 2 }}{2} + 0} \right) = \frac{{\sqrt 2 }}{4}\end{array}\)

\(M=\left(\dfrac{1}{2}x-2\right)^3=\left(12-2\right)^3=1000\)

B=1-2-3+4+5-6-7+8+..........+21-22-23+24

B=(1-2-3+4)+(5-6-7+8)+.......+(21-22-23+24)

B=0+0+............+0

B=0

\(G=\dfrac{7}{8}+\dfrac{11}{24}+\dfrac{5}{8}+\dfrac{23}{24}+\dfrac{11}{8}+\dfrac{13}{24}\\ \Rightarrow G=\left(\dfrac{7}{8}+\dfrac{5}{8}+\dfrac{11}{8}\right)+\left(\dfrac{11}{24}+\dfrac{23}{24}+\dfrac{13}{24}\right)\\ \Rightarrow G=\dfrac{23}{8}+\dfrac{47}{24}\\ \Rightarrow G=\dfrac{69}{24}+\dfrac{47}{24}\\ \Rightarrow G=\dfrac{116}{24}\\ \Rightarrow G=\dfrac{29}{6}\)

hay quá mà mik còn quá nhiều

\(G=\dfrac{9}{8}+\dfrac{23}{24}+\dfrac{1}{8}+\dfrac{17}{24}+\dfrac{13}{8}+\dfrac{19}{24}\\ \Rightarrow G=\left(\dfrac{9}{8}+\dfrac{1}{8}+\dfrac{13}{8}\right)+\left(\dfrac{23}{24}+\dfrac{17}{24}+\dfrac{19}{24}\right)\\ \Rightarrow G=\dfrac{23}{8}+\dfrac{59}{24}\\ \Rightarrow G=\dfrac{69}{24}+\dfrac{59}{24}\\ \Rightarrow G=\dfrac{128}{24}\\ \Rightarrow G=\dfrac{16}{3}\)

\(=\left(\dfrac{9}{8}+\dfrac{1}{8}+\dfrac{13}{8}\right)+\left(\dfrac{23}{24}+\dfrac{17}{24}+\dfrac{19}{24}\right)\)

\(=\dfrac{23}{8}+\dfrac{59}{24}\)

\(=\dfrac{16}{3}\)