Số tiền mua đường là:

\(15\times20000=300000\) (đồng)

Số tờ tiền bác An phải đưa là:

\(300000\div100000=3\) (tờ)

\(x-\dfrac{1}{1.2}-\dfrac{1}{2.3}-\dfrac{1}{3.4}-...-\dfrac{1}{98.99}-\dfrac{1}{99.100}=\dfrac{1}{100}\)

\(\Leftrightarrow x-\left(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{99.100}\right)=\dfrac{1}{100}\)

\(\Leftrightarrow x-\left(\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{99}-\dfrac{1}{100}\right)=\dfrac{1}{100}\)

\(\Leftrightarrow x-\left(1-\dfrac{1}{100}\right)=\dfrac{1}{100}\)

\(\Leftrightarrow x=\left(1-\dfrac{1}{100}\right)+\dfrac{1}{100}\)

\(\Leftrightarrow x=1\)

`(-3)/14 + 2/21 = (-9)/42 + 4/42 =(-9+4)/42=(-5)/42`

`2/6-(-2/3)+7/4=2/6+2/3+7/4=4/12+8/12+21/12=(4+8+21)/12=33/12=11/4`

`1/12 - (1/6-1/4)=1/12-1/6+1/4=1/12-2/12+3/12=(1-2+3)/12=2/12 = 1/6`

`3/7+ (-9/5)-4/3=3/7-9/5-4/3=45/105 - 189/105 - 140/105 = (45-189-140)/105 =(-284)/105`

a, -3/14+2/21 = -5/42

b,2/6- -2/3+7/4 = 1 + 7/4= 11/4 

c,1/12-(1/6-1/4) = 1/12 -  ( -1/12) = 1/6 

d,3/7+(-9/5)-4/3 = -22/35 - 4/3 =  -4/105

`3.125.121.8`

`=(8.125).121.3`

`=1000.121.3`

`=121000.3`

`=363000`

3.125.121.8

= ( 3.125 ) . (121.8)

= ( 3.121 ) . (125.8)

= 363 . 1000

= 363000

\(\left[\left(321-797\right)+\left(-321+697\right)\right]-371-\left[1900+\left(-371\right)\right]\)

\(=321-797-321+697-371-1900+371\)

\(=\left(321-321\right)-\left(797-697\right)+\left(-371+371\right)-1900\)

\(=-100-1900\)

\(=-2000\)

=vân vân và may may    

mấy bài lặp lại thì xoá đi e :v

Ũa ;-;

cùng `1` bạn hả cj 

`(14-12-7)-[-(-3+2+5+9)]`

`= (14-12-7)- [3-2-5-9]`

`=(2-7)-(1-5-9)`

`=-5-(-13)`

`=-5+13`

`=8`

thế Phú ra đáp số là bao nhiêu? đưa đây anh chấm cho