khó thế!

mới lớp 5 ko giải được

X = 3^a.9^b

X = 3^a.(3²)^b

X = 3^a.3^2b

X = 3^a+2b

A = \(\dfrac{1}{1.2}\) + \(\dfrac{1}{3.4}\) + \(\dfrac{1}{5.6}\) + ... + \(\dfrac{1}{49.50}\)

A <  \(\dfrac{1}{1.2}\) + \(\dfrac{1}{2.3}\) + \(\dfrac{1}{3.4}\) + \(\dfrac{1}{4.5}\) + \(\dfrac{1}{5.6}\) + ... + \(\dfrac{1}{49.50}\)

A < \(\dfrac{1}{1}\) - \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{4}\) - \(\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{49}-\dfrac{1}{50}\)

A < \(\dfrac{1}{1}-\dfrac{1}{50}\) < 1

A < 1 (đpcm)

Có vẻ biểu thức A lỗi rồi. Bạn xem lại nhé.

\(\overline{a,b}-\overline{0,ab}=4,05\)

=>\(\overline{a,b}-\overline{a,b}\times0,1=4,05\)

=>\(0,9\times\overline{a,b}=4,05\)

=>\(\overline{a,b}=4,05:0,9=4,5\)

\(\overline{a,b}\) - \(\overline{0,ab}\) = 4,05

\(\overline{a,b}\) x 1 - \(\overline{a,b}\)  x 0,1 = 4,05

\(\overline{a,b}\) x (1 - 0,1) = 4,05

\(\overline{a,b}\) x 0,9 = 4,05

\(\overline{a,b}\)         = 4,05 : 0,9

\(\overline{a,b}\)         = 4,5

Em xem lại đề nhé. Các phân số này không có quy luật nên không tính được

1: \(\dfrac{1}{26}+\dfrac{-5}{39}=\dfrac{3}{78}-\dfrac{10}{78}=\dfrac{-7}{78}\)

2: \(-\dfrac{1}{16}+\dfrac{-1}{24}=\dfrac{-3}{48}+\dfrac{-2}{48}=-\dfrac{5}{48}\)

3: \(\dfrac{4}{7}-\left(-\dfrac{1}{9}\right)=\dfrac{4}{7}+\dfrac{1}{9}=\dfrac{36}{63}+\dfrac{7}{63}=\dfrac{43}{63}\)

4: \(-0,16+\dfrac{-3}{2}=-0,16-1,5=-1,66\)

5: \(\dfrac{3}{5}-\left(-\dfrac{2}{7}\right)=\dfrac{3}{5}+\dfrac{2}{7}=\dfrac{21}{35}+\dfrac{10}{35}=\dfrac{31}{35}\)

6: \(-\dfrac{4}{12}-\left(-\dfrac{13}{39}-0,25\right)\)

\(=-\dfrac{1}{3}+\dfrac{1}{3}+0,25=0,25\)

7: \(\dfrac{2}{5}-\left(\dfrac{4}{3}+\dfrac{4}{5}\right)-\left(-\dfrac{1}{9}-0,4\right)+\dfrac{11}{9}\)

\(=\dfrac{2}{5}-\dfrac{4}{3}-\dfrac{4}{5}+\dfrac{1}{9}+\dfrac{2}{5}+\dfrac{11}{9}\)

\(=\dfrac{12}{9}-\dfrac{4}{3}=0\)

1. \(\dfrac{1}{26}+\dfrac{-5}{39}=\dfrac{3}{78}+\dfrac{-10}{78}=\dfrac{3-10}{78}=\dfrac{-7}{78}\)

2. \(\dfrac{-1}{16}+\dfrac{-1}{24}=\dfrac{-3}{48}+\dfrac{-2}{48}=\dfrac{-3-2}{48}=\dfrac{-5}{48}\)

3. \(\dfrac{4}{7}-\left(\dfrac{-1}{9}\right)=\dfrac{4}{7}+\dfrac{1}{9}=\dfrac{36}{63}+\dfrac{7}{63}=\dfrac{36+7}{63}=\dfrac{43}{63}\)

4. \(-0.16+\dfrac{-3}{2}=\dfrac{-4}{25}+-\dfrac{3}{2}=\dfrac{-8-75}{50}=\dfrac{-83}{50}\)

5. \(\dfrac{3}{5}-\left(-\dfrac{2}{7}\right)=\dfrac{3}{5}+\dfrac{2}{7}=\dfrac{21}{35}+\dfrac{10}{35}=\dfrac{21+10}{35}=\dfrac{31}{35}\)

6. \(\dfrac{-4}{12}-\left(\dfrac{-13}{39}-0,25\right)+0,75=\dfrac{-1}{3}+\dfrac{1}{3}+0,25+0,75=\left(\dfrac{-1}{3}+\dfrac{1}{3}\right)+\left(0,25+0,75\right)=0+1\)

7. \(2,5-\left(\dfrac{4}{3}+\dfrac{4}{5}\right)-\left(\dfrac{-1}{9}-0,4\right)+\dfrac{11}{9}\)

\(=\dfrac{5}{2}-\dfrac{4}{3}-\dfrac{4}{5}+\dfrac{1}{9}+\dfrac{2}{5}+\dfrac{11}{9}\)

\(=\dfrac{5}{2}-\dfrac{4}{3}-\dfrac{4}{5}+\left(\dfrac{1}{9}+\dfrac{11}{9}\right)+\dfrac{2}{5}\)

\(=\dfrac{5}{2}-\dfrac{4}{3}+\left(\dfrac{2}{5}-\dfrac{4}{5}\right)+\dfrac{12}{9}+\dfrac{2}{5}\)

\(=\dfrac{5}{2}-\dfrac{4}{3}-\dfrac{2}{5}+\dfrac{4}{3}+\dfrac{2}{5}\)

\(=\dfrac{5}{2}\)

Gọi số cần tìm là ab

Ta có :

a0b = 9 . ab

100a + b = 9 . ( 10a + b )

100a + b = 90a + b 

100a - b = 9b - b

10a = 8b

a = 8 : 10b = $\frac{4}{5}$b 

Mà a,b là số tự nhiên có 1 chữ số.

=> a = 4 ; b = 5

Vậy số cần tìm là 45

\(1,-\dfrac{4}{5}-\dfrac{2}{9}\)

\(=-\dfrac{46}{45}\)

\(2,-\dfrac{1}{9}-\dfrac{5}{12}\)

\(=-\dfrac{4}{36}-\dfrac{15}{36}=-\dfrac{19}{36}\)

\(3.4,5-\left(-\dfrac{7}{5}\right)\)

\(=4,5+\dfrac{7}{5}\)

\(=4,5+1,4=5,9\)

\(4.-0,8+-\dfrac{16}{5}\)

\(=-0,8+-3,2\)

\(=-4\)

\(5,-\dfrac{5}{17}+\dfrac{13}{34}\)

\(=-\dfrac{10}{34}+\dfrac{13}{34}\)

\(=\dfrac{3}{34}\)

a:

Sửa đề Tìm giá trị nhỏ nhất để A là số nguyên

 \(A=\dfrac{4-5x}{x+3}=\dfrac{-5x+4}{x+3}\)

\(=\dfrac{-5x-15+19}{x+3}=-5+\dfrac{19}{x+3}\)

Để A min thì x+3=-1

=>x=-4

b: \(\left(x-3\right)^2+5>=5\forall x\)

\(\left(x^2-9\right)^2+5>=5\forall x\)

=>\(\dfrac{25}{\left(x^2-9\right)^2+5}< =\dfrac{25}{5}=5\forall x\)

Dấu '=' xảy ra khi \(\left\{{}\begin{matrix}x-3=0\\x^2-9=0\end{matrix}\right.\)

=>x=3

b: \(-3-\left(-\dfrac{3}{4}\right)=-3+\dfrac{3}{4}=-\dfrac{12}{4}+\dfrac{3}{4}=-\dfrac{9}{4}\)

c: \(\dfrac{24}{126}-\left(-\dfrac{5}{28}\right)=\dfrac{4}{21}+\dfrac{5}{28}=\dfrac{16}{84}+\dfrac{15}{84}=\dfrac{31}{84}\)

8: \(-\dfrac{14}{20}+0,6=-0,7+0,6=-0,1\)

\(b,-3-\left(-\dfrac{3}{4}\right)\)              \(c,\dfrac{24}{126}-\left(-\dfrac{5}{28}\right)\)

\(=-3+\dfrac{3}{4}\)                      \(=\dfrac{24}{126}+\dfrac{5}{28}\)

\(=-\dfrac{9}{4}\)                            \(=\dfrac{4}{21}+\dfrac{5}{28}\)

                                      \(=\dfrac{31}{84}\)

\(d,-\dfrac{14}{20}+0,6\)

\(=-\dfrac{7}{10}+\dfrac{6}{10}\)

\(=-\dfrac{1}{10}=-0,1\)