Còn cần ko nhỉ, mấy câu này cơ bản áp dụng công thức thôi mà?

1 + 3 - 5 - 7 + 9 + 11 - 13 - 17 + .....+ 393 + 395 - 397 - 399

có (399-1) : 2 + 1 = 200 số

= (1+3-5-7) + (9+11-13-15) + ..... + (393 + 395 - 397 - 399)

= (-8) + (-8) + ... + (-8) 

có 200 : 4 = 50 số -8

= (-8) x 50

= -400

hinh nhu sai de-_-

SSH: (399-1):2+1= 200
Neu chia moi nhom 4 so thi so cap so la:
200:4 = 50
Ta co:
B=1+3-5-7+9+11-13-15+...+393+395-397-399
B= (1+3-5-7)+(9+11-13-15)+...+(393+395-397-399)
B= -8 + -8 +...+ -8
B= -8 . 50
B= -400

\(K=\left(-1\frac{1}{2}\right)\left(-1\frac{1}{3}\right)...\left(-1\frac{1}{999}\right)=\left(-1\right)^{998}1\frac{1}{2}\cdot1\frac{1}{3}\cdot...\cdot1\frac{1}{999}\)

\(K=\frac{3}{2}\cdot\frac{4}{3}\cdot...\cdot\frac{1000}{999}=\frac{\left(3.4...999\right)\cdot1000}{2\cdot\left(3.4...999\right)}=\frac{1000}{2}=500\)

Chúc bạn học tốt!

(1-1/3).(1-1/6).(1-1/10)...(1-1/780) (ko có dấu "...") 
Giải: (1-1/3).(1-1/6).(1-1/10)...(1-1/780) 
= 2/3 . 5/6....779/780 
= 4/6 . 10/12.....1558/1560 
= 1.4 . 2.5 .... 38.41/ 2.3 . 3. 4. .....39.40 
= ( 1.2.3....38).(4.5....41)/(2.3.4....39)(3... 
triệt tiêu xong còn 41/39.3= 41/117 
ĐS = 41/117

1,75 phút nha

Lấy  45 : 60 + 1 = 1,75 phút nha

D = $\frac{2}{3}.\frac{5}{6}.\frac{9}{10}. ... .\frac{799}{780}$

   = $\frac{2.2}{2.3}.\frac{2.5}{3.4}.\frac{3.6}{4.5}. ... .\frac{38.41}{39.40}$

   = $\frac{2.2}{2.3}.\frac{2.3. ... .38}{3.4. ... 39}.\frac{5.6. ... .41}{4.5. ... .40}$

   = $\frac{2}{3}.\frac{2}{39}.\frac{41}{4}$

   = $\frac{41}{3.39}$

D = \(\frac{2}{3}.\frac{5}{6}.\frac{9}{10}.\frac{14}{15}.....\frac{779}{780}\)

   = \(\frac{2.2}{2.3}.\frac{2.5}{3.4}.\frac{3.6}{4.5}.\frac{4.7}{5.6}.....\frac{38.41}{39.40}\)

   = \(\frac{2}{3}.\frac{2.3.4....38}{3.4.5....39}.\frac{5.6.7.....41}{4.5.6.....40}\)

   = \(\frac{2}{3}.\frac{2}{39}.\frac{41}{4}\)

   = \(\frac{41}{117}\)

\(\left(\frac{x}{2}-\frac{1}{3}\right):\frac{1}{2}=\left(\frac{1}{4}-\frac{3}{2}\right):\left(1-\frac{5}{4}\right)\)

\(\left(\frac{3x-2}{6}\right):\frac{1}{2}=\left(-\frac{5}{4}\right):\left(-\frac{1}{4}\right)\)

\(\left(\frac{3x-2}{6}\right):\frac{1}{2}=5\)

\(\left(\frac{3x-2}{6}\right)=\frac{5}{2}\)

           Áp dụng công thức \(\frac{a}{b}=\frac{c}{d}\Rightarrow ad=bc\) ta đc:

                    \(\Rightarrow2\left(3x-2\right)=30\)

                    \(\Rightarrow\left(3x-2\right)=15\)

                    \(\Rightarrow3x=17\)

                         \(\Rightarrow x=\frac{17}{3}\)

\(\left(\frac{x}{2}-\frac{1}{3}\right):\frac{1}{2}=\left(\frac{1}{4}-\frac{3}{2}\right):\left(\frac{1-5}{4}\right)\)

\(\left(\frac{x}{2}-\frac{1}{3}\right):\frac{1}{2}=\left(\frac{1}{4}-\frac{6}{4}\right):1\)

\(\left(\frac{x}{2}-\frac{1}{3}\right):\frac{1}{2}=-\frac{5}{4}:1\)

\(\left(\frac{x}{2}-\frac{1}{3}\right):\frac{1}{2}=-\frac{5}{4}\)

\(\frac{x}{2}-\frac{1}{3}=-\frac{5}{4}\times\frac{1}{2}\)

\(\frac{x}{2}-\frac{1}{3}=-\frac{5}{8}\)

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

\(\frac{x}{2}=-\frac{7}{24}\)

\(x\times24=-14\)

\(x=-\frac{7}{12}\)

Hình như có gì đó sai sai Chị ah