Xét ΔDEF có \(\frac{EF}{\sin D}=\frac{DF}{\sin E}\)

=>\(\frac{2.8}{\sin D}=\frac{4.2}{\sin30}=4,2:\frac12=8,4\)

=>sin D=1/3

=>\(\hat{D}\) ≃19 độ

Xét ΔDEF có \(\hat{D}+\hat{E}+\hat{F}=180^0\)

=>\(\hat{F}=180^0-30^0-19^0=131^0\)

Xét ΔDEF có \(\frac{DE}{\sin F}=\frac{DF}{\sin E}\)

=>\(\frac{DF}{\sin131}=\frac{4.2}{\sin30}=8,4\)

=>\(DF=8,4\cdot\sin131\) ≃6,34(cm)

1/6*3+1/6*9+1/9*12+........+1/30*33

=(1/3-1/6)+(1/6-1/9)+(1/9-1/12)+........+(1/30-1/33)

=1/3-1/6+1/6-1/9+1/9-1/12+........+1/30-1/33

=1/3-1/33

=10/33

nho k cho mink nha 

CHUC BAN HOC GIOI !

Gợi ý: 18 = 3.6

           54 = 6.9

           108 = 9.12

           .............

           990 = 30.33

Gấp 3 lần R rồi dùng sai phân hữu hạn.

Tự làm tiếp nhé!!!

F = 1/3.6 + 1/6.9 + 1/9.12 + ... + 1/30.33

F = 1/3.(1.3-1/6+1/6-1/9+1/9-1/12+...+1/30-1/33)

F = 1/3.(1/3-1/33)

F = 1/3.10/33

F = 10/99

F = 1/ 3.6 + 1/ 6.9 + 1/ 9.12 +...+1/ 30.33

F = 1/3 . (  1/3 -1/6 + 1/6 - 1/9 + 1/9 - 1/12 + ... + 1/30 - 1/33 )

F = 1/3 . ( 1/3 - 1/33 )

F = 1/3 . 10/ 33

F = 10 /99

\(F=\frac{1}{18}+\frac{1}{54}+\frac{1}{108}+...+\frac{1}{990}\)

\(F=\frac{1}{9}\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{110}\right)\)

\(F=\frac{1}{9}\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{10.11}\right)\)

\(F=\frac{1}{9}\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{10}-\frac{1}{11}\right)\)

\(F=\frac{1}{9}\left(1-\frac{1}{11}\right)\)

\(F=\frac{1}{9}.\frac{10}{11}=\frac{10}{99}\)

\(\frac{7}{8}\)- y = \(\frac{1}{2}\)+ \(\frac{1}{8}\)

\(\frac{7}{8}\)- y = \(\frac{5}{8}\)

          y = \(\frac{7}{8}\)- \(\frac{5}{8}\)

          y = \(\frac{1}{4}\)

\(\frac{7}{8}-y=\frac{1}{2}+\frac{1}{8}\)

\(\Rightarrow\frac{7}{8}-y=\frac{5}{8}\)

\(\Rightarrow y=\frac{7}{8}-\frac{5}{8}=\frac{2}{8}=\frac{1}{4}\)

Vậy \(y=\frac{1}{4}\)

Ta có: \(A=1-2+3-4+5-6+7-8+9\)

\(=(1+9)-(2+8)+(3+7)-(4+6)+5\)

\(=10-10+10-10+5\)

\(=5\)

Vậy  \(A=5\) 

B = 12 - 14 + 16 - 18 + ... + 2008 - 2010

B = -2 + (-2)+ (-2)+ (-2) + ...+ (-2)

B = -2 . 100

B = -200

lx + 12l + 21 = (-7).(-8)

lx + 12l + 21 =  56

lx + 12l         = 56 - 21

lx + 12l         =  35

Suy ra x + 12 = 35 hay x + 12 = -35

TH1: 

x + 12 = 35

x         = 35 - 12

x         = 23

TH2:

x + 12 = -35

x         = -35 - 12

x         = -35 + (-12)

x         =     -47

k mk nha bn,ủng hộ mk nha

A = 1/20

B = 85/504

               k cho mình nha 

giải thích giúp mình đi Bui Tra My