2. BCNN(15,88)=1320

BCNN(45,120,36)=360

BCNN(28,30,98)=2940

Bài 3: 

3=3

8=2³

16=2⁴

84=2²x3x7

120=2³x3x5

145=5x29

Cách cho ta phân số là:\(\dfrac{3}{-11}\);\(\dfrac{0}{3}\)

8:0,25=

\(\frac{3}{5}+\frac{-1}{25}-\frac{7}{20}\)

\(=\frac{3}{5}-\frac{1}{25}-\frac{7}{20}\)

\(=\frac{60}{100}-\frac{4}{100}-\frac{35}{100}\)

\(=\frac{21}{100}\)

\(\frac{3}{5}+\frac{-1}{25}-\frac{7}{20}\)

\(=\frac{60}{100}-\frac{4}{100}-\frac{35}{100}\)

\(=\frac{60-4-35}{100}\)

\(=\frac{21}{100}\)

P/S: hok tốt nhé

\(50\%x+\frac{2}{3}x=x-5\)

\(\Rightarrow\frac{1}{2}x+\frac{2}{3}x=x-5\)

\(\Rightarrow x\left(\frac{1}{2}+\frac{2}{3}\right)=x-5\)

\(\Rightarrow x.\frac{7}{6}=x-5\)

\(\Rightarrow x-\frac{7}{6}x=5\)

\(\Rightarrow\frac{-x}{6}=5\Leftrightarrow-x=30\Leftrightarrow x=-30\)

\(50\%x+\frac{2}{3}x=x-5\)

\(\frac{1}{2}x+\frac{2}{3}x=x-5\)

\(x\left(\frac{1}{2}+\frac{2}{3}\right)=x-5\)

\(x.\frac{7}{6}=x-5\)

\(x.\frac{7}{6}-x=-5\)

\(x.\frac{1}{6}=-5\)

\(x=\left(-5\right):\frac{1}{6}=-30\)

Vậy x= -30

Câu 5:

\(A=1+3+3^2+3^3+...+3^{19}+3^{20}\)

\(\Leftrightarrow3A=3+3^2+3^3+3^4+...+3^{20}+3^{21}\)

\(\Rightarrow3A-A=3^{21}-1\)

\(\Rightarrow2A=3^{21}-1\)

\(\Rightarrow A=\dfrac{3^{21}-1}{2}\)

Câu 3: 

c: Trường hợp 1: n=2k

\(\Leftrightarrow n\left(n+2017\right)=2k\cdot\left(2k+2017\right)⋮2\)

Trường hợp 1: n=2k+1

\(\Leftrightarrow n\left(n+2017\right)=\left(2k+1\right)\left(2k+2018\right)⋮2\)

\(a,25.69+31.25-150\\ =25.\left(69+31\right)-150\\ =25.100-150\\ =2500-150\\ =2350\)

\(b,198:\left[130-\left(27-19\right)^2\right]+2021^0\\ =198:\left(130-8^2\right)+1\\ =198:\left(130-64\right)+1\\ =198:66+1\\ =3+1\\ =4\)

\(c,5^{20}:\left(5^{15}.15+5^{15}.10\right)\\ =5^{20}:\left[5^{15}.\left(15+10\right)\right]\\ =5^{20}:\left(5^{15}.25\right)\\ =5^{20}:\left(5^{15}.5^2\right)\\ =5^{20}:5^{17}\\ =5^3\\ =125\)