Đặt A=\(\dfrac{4^2}{20.24}+\dfrac{4^2}{24.28}+...+\dfrac{4^2}{76.80}\)

A=\(\dfrac{16}{20.24}+\dfrac{16}{24.28}+...+\dfrac{16}{76.80}\)

A=4.[\(\dfrac{4}{20.24}+\dfrac{4}{24.28}+...+\dfrac{4}{76.80}\)]

A=4.\(\left[\dfrac{1}{20}-\dfrac{1}{24}+\dfrac{1}{24}-\dfrac{1}{28}+...+\dfrac{1}{76}-\dfrac{1}{80}\right]\)

A=4.\(\left[\dfrac{1}{20}+\dfrac{1}{24}-\dfrac{1}{24}+\dfrac{1}{28}-\dfrac{1}{28}+...+\dfrac{1}{78}-\dfrac{1}{78}-\dfrac{1}{80}\right]\)

A=4.\(\left[\dfrac{1}{20}-\dfrac{1}{80}\right]\)

A=4.\(\dfrac{3}{80}\)

A=\(\dfrac{3}{20}\)<1

=>A<1

Tick mink nha

Ta có: \(\frac{4^2}{20.24}+\frac{4^2}{24.28}+...+\frac{4^2}{76.80}\)

\(=4.\left(\frac{4}{20.24}+\frac{4}{24.28}+...+\frac{4}{76.80}\right)\)

\(=4.\left(\frac{1}{20}-\frac{1}{24}+\frac{1}{24}-\frac{1}{28}+...+\frac{1}{76}-\frac{1}{80}\right)\)

\(=4.\left(\frac{1}{20}-\frac{1}{80}\right)=4.\frac{3}{80}=\frac{3}{20}< 1\)

Vậy \(\frac{4^2}{20.24}+\frac{4^2}{24.28}+...+\frac{4^2}{76.80}< 1\)

28 x 76 + 24 x 28 - 28 x 20

= 28 x (76 + 24 - 20)

= 28 x 80

= 2240

Ta có : 28 . 76 + 24 . 28 - 28 . 20 = 28 . ( 76 + 24 - 20 )

                                                = 28 .        80

                                                =     2240

=2240

bạn nhé

tk nha@@@@@@@@@@@@@@@@@@@@

LOL

28.76+24.28-28.20

=53,04-28,20

=24,84

Học sinh nhẩm và ghi kết quả như sau:

35 : 5 = 7

28 : 7 = 4

24 : 6 = 4

21 : 3 = 7

35 : 7 = 5

28 : 4 = 7

24 : 4 = 6

21 : 7 = 3

35 : 5 = 7

28 : 7 = 4

24 : 6 = 4

21 : 3 = 7

35 : 7 = 5

28 : 4 = 7

24 : 4 = 6

21 : 7 = 3

Giải cho bạn 2 bài tìm x nhé! Bài kia dễ,tự giải

a) \(3^{x+2}-3^x=8.243\Leftrightarrow3^x.3^2-3^x=1944\)

\(\Leftrightarrow3^x\left(3^2-1\right)=1944\Leftrightarrow3^x.8=1944\)

\(\Leftrightarrow3^x=243=3^5\Leftrightarrow x=5\)

b) \(1+2+3+...+x=210\)

\(\Leftrightarrow\frac{x\left(x+1\right)}{2}=210\Leftrightarrow x\left(x+1\right)=420\)

\(\Leftrightarrow20\left(20+1\right)=420\Rightarrow x=20\)

thank ^ - ^

Bạn giải câu 3 ^x+2 - 3^x = 8.243 mk mới để ý vẫn còn cách giải nữa = 3 =

...

nhanh nhất thì tick cho

Nguu Quá

a) 57 + 28 = 85

24 + 67 = 91

46 + 39 = 85

b) 83 - 19 = 64

42 - 38 = 4

90 - 76 = 14

\(\begin{array}{*{20}{c}}{a)\,\,}\\{}\\{}\end{array}\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{57}\\{28}\end{array}}\\\hline{\,\,\,\,85}\end{array}\)                            \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{24}\\{67}\end{array}}\\\hline{\,\,\,\,91}\end{array}\)                            \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{46}\\{39}\end{array}}\\\hline{\,\,\,\,85}\end{array}\)

\(\begin{array}{*{20}{c}}{b)\,\,}\\{}\\{}\end{array}\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{83}\\{19}\end{array}}\\\hline{\,\,\,\,64}\end{array}\)                           \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{42}\\{38}\end{array}}\\\hline{\,\,\,\,04}\end{array}\)                            \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{90}\\{76}\end{array}}\\\hline{\,\,\,\,14}\end{array}\)