Bài 1:Tính hợp lí

a)-12,5+17,55-3,5+2,45

b)0,175-(\(2\dfrac{1}{3}\)+0,175)

c)\(\dfrac{5}{13}\).\(\dfrac{-3}{10}\)+\(\dfrac{3}{10}\).\(\dfrac{-8}{13}\)+(-0,7)

Bài 2:Tìm x biết

a)x+\(\dfrac{2}{5}\)=2,4

b)2x-\(\dfrac{4}{5}\)=-1,5

c)11-(15+11)=x-(25-9)

Bài 3:Cho A=\(\dfrac{1}{2^2}\)+\(\dfrac{1}{3^2}\)+\(\dfrac{1}{4^2}\)+\(\dfrac{1}{5^2}\)+....+\(\dfrac{1}{100^2}\)

         Chứng tỏ A<1

Bài 3 : Tính 

a) \(\dfrac{2}{5}+\dfrac{11}{15}\)             b) \(\dfrac{2}{7}-\dfrac{2}{11}\)            c) \(\dfrac{11}{13}\) x \(\dfrac{26}{31}\)            d) \(\dfrac{1}{2}:\dfrac{1}{3}\) x \(\dfrac{2}{5}\)

giúp mik với , giải rõ ràng ra nha . ( đùưng vào nói ltinh gì zới mik nha ) 

Bài 15:

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

b)\(\dfrac{-3}{7}\) - \(\dfrac{4}{7}\):x = -2

Bài 16

a) x - \(\dfrac{10}{3}\) = \(\dfrac{7}{15}\) . \(\dfrac{3}{5}\)

b) x + \(\dfrac{3}{22}\)= \(\dfrac{27}{121}\) . \(\dfrac{11}{9}\)

c) \(\dfrac{8}{23}\) . \(\dfrac{48}{24}\) - x = \(\dfrac{1}{3}\)

d) 1 - x = \(\dfrac{49}{65}\).\(\dfrac{5}{7}\)

Bài 17: tìm x

a) \(\dfrac{62}{7}\) . x = \(\dfrac{29}{9}\): \(\dfrac{3}{56}\)

b) \(\dfrac{1}{5}\) : x=\(\dfrac{1}{5}\)+\(\dfrac{1}{7}\)

bài 18:

a)\(\dfrac{2}{5}\)+\(\dfrac{3}{4}\): x =\(\dfrac{-1}{2}\)

b)\(\dfrac{5}{7}\) - \(\dfrac{2}{3}\) . x = \(\dfrac{4}{5}\)

c) \(\dfrac{1}{2}\)x + \(\dfrac{3}{5}\)x = \(\dfrac{-2}{3}\)

d) \(\dfrac{4}{7}\).x-x = \(\dfrac{-9}{14}\)

bài 19: tính 

\(\dfrac{1}{1.2}\)+\(\dfrac{1}{2.3}\)+\(\dfrac{1}{3.4}\)+...+ \(\dfrac{1}{2018.2019}\)

bài 20:tìm x 

\(\dfrac{1}{1.2}\)+\(\dfrac{1}{2.3}\)+...+\(\dfrac{1}{x.\left(x+1\right)}\)=\(\dfrac{2008}{2009}\)

bài 21: tìm x

\(\dfrac{x+1}{99}\)+\(\dfrac{x+2}{98}\)\(\dfrac{x+3}{97}\)\(\dfrac{x+4}{96}\)=-4

bài 22 : so sánh các phân số sau:

a) \(\dfrac{-1}{5}\)+\(\dfrac{4}{-5}\)và 1

b) \(\dfrac{3}{5}\) và \(\dfrac{2}{3}\)+\(\dfrac{-1}{5}\)

c)\(\dfrac{3}{2}\)+\(\dfrac{-4}{3}\) và \(\dfrac{1}{10}\)+\(\dfrac{-4}{5}\)

d) \(\dfrac{1}{2}\)+\(\dfrac{1}{3}\)+\(\dfrac{1}{4}\)+\(\dfrac{1}{5}\)+\(\dfrac{1}{6}\) và 2

bài 7 tìm những giá trị nhuyên dương X thỏa mãn 

3) \(\dfrac{-5}{11}\)<\(\dfrac{9}{x}\)<\(\dfrac{-5}{12}\)

4) \(\dfrac{-11}{13}\)<\(\dfrac{9}{x}\)<\(\dfrac{-11}{15}\)

5) \(\dfrac{-4}{5}\)<\(\dfrac{9}{x}\)<\(\dfrac{-4}{7}\)

nhanh cần gấp nhé 

Bài 1 : thực hiện các phép tính ( tính hợp lí nếu có thể ):
a ,\(\dfrac{7}{2}.\dfrac{8}{13}+\dfrac{8}{13}.\dfrac{-5}{2}+\dfrac{8}{13}\)
b , \(\dfrac{-5}{17}.\dfrac{-9}{23}+\dfrac{9}{23}.\dfrac{-22}{17}+11\dfrac{9}{23}\)
Bài 2 : tìm x , biết :
a , \(\dfrac{7}{8}+x=\dfrac{3}{5}\)
b , \(\dfrac{1}{3}:\left(2x-1\right)=\dfrac{-4}{24}\)