Có A= 2.1+ 2.3+2.5+........+2. 97+ 2.99 

=>A=2 . (1+3+5+.....+97+99)

=>A=2 . [(1+99)+(3+97)+(5+95)+.....+(50+50)]

=>A=2 . [100+100+100+....+100]   Có 25 số 100

=>A=2 . [100.25]

=>A= 2 . 2500

=>A=5000

Câu b bạn làm tương tự nha

1.  25 : 5,7 = 250/57

2.  30:2.8.4 = 480

3.  20:2^2.14= 70

4.  125:5^3.170= 170

5.  64:2^5.30.4=240

6.  (25:5^2.30): 15.7=14

       bạn à! Nhiều quá mình ko làm hết được. sorry nha.^-^                     

dù dài quá nhưng cố  gắng giúp mình làm hết nha

\(A=1.2+2.3+3.4+...+99.100\)

\(\Rightarrow3A=1.2.3+2.3\left(4-1\right)+3.4\left(5-2\right)+...+90.100\left(101-98\right)\)

\(\Rightarrow3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+99.100.101-98.99.100\)

\(\Rightarrow3A=99.100.101\)

\(\Rightarrow A=\left(99.100.101\right):3\)

\(\Rightarrow A=333300\)

\(B=1.3+2.4+3.5+...+99.101\)

\(\Rightarrow B=1\left(2+1\right)+2\left(3+1\right)+3\left(4+1\right)+...+99\left(100+1\right)\)

\(\Rightarrow B=1.2+1+2.3+2+3.4+3+...+99.100+99\)

\(\Rightarrow B=\left(1.2+2.3+3.4+...+99.100\right)+\left(1+2+3+...+99\right)\)

\(\Rightarrow B=333300+4950\)

\(\Rightarrow B=338250\)

kết bạn kiểu gì

a)1+3+5+7+9+...+x=1600

=>[(x-1):2+1].(x+1)/2=1600

=>(1/2.x-1/2+1).(x+1)=1600:1/2

=>(1/2.x-1/2+1).(x+1)=3200

=>(x+1)².1/2=3200

=>(x+1)²       =3200:1/2

=>(x+1)²=6400

=>x+1=80

=>x=80-1=79

Ta có:

\(\frac{2^3.3^4}{2^2.3^2.5}=\frac{2.3^2}{5}=\frac{18}{5}\)

\(\frac{2^4.5^2.11^2.7}{2^3.5^3.7^2.11}=\frac{2.11}{5.7}=\frac{22}{35}\)

THANKS

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{99.100}\)

= \(2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\right)\)

= \(2.\left(1-\frac{1}{100}\right)\)

= \(2.\frac{99}{100}\)

= \(\frac{99}{50}\)

đặt 2 ra ngoài