a) 76589 + 254 x 68 - 68975 =76589+17272-68975

                                              =162836

b) 1748 x 32 + 9900 :495 =55936+205

                                         =55956

chỗ bài b không phải 205 đâu mà là 20

a) 76589 + 254 x 68 - 68975

= 76589 + 17272 - 68975

= 93861 - 68975

= 24886

b) 1748 x 32 + 9900 : 495

= 55936 + 20 

= 55956

Giải thích các bước giải:

9900 : 36 - 15 x 11 

= 275 - 165

= 110

9700 : 100 + 36 x 12

= 97 + 432

= 529

( 15792 : 336 ) x 5 + 27 x 11

= 47 x 5 + 297

= 235 + 297

= 532

( 160 x 5 - 25 x 4 ) : 4

= ( 800 - 100 ) : 4

= 700 : 4

= 175

1995 x 253 + 8910 : 495 = 504735 + 18 = 504753

A = \(\dfrac{1}{12}\)+ \(\dfrac{1}{20}\)+ \(\dfrac{1}{30}\)+...+\(\dfrac{1}{9900}\)

A = \(\dfrac{1}{3\times4}\)+ \(\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+...+\dfrac{1}{99\times100}\)

A = \(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{99}-\dfrac{1}{100}\)

A = \(\dfrac{1}{3}\) - \(\dfrac{1}{100}\)

A = \(\dfrac{97}{300}\) 

Lời giải:

Gọi tổng trên là $A$

$A=\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+....+\frac{1}{99.100}$

$=\frac{4-3}{3.4}+\frac{5-4}{4.5}+\frac{6-5}{5.6}+...+\frac{100-99}{99.100}$

$=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+....+\frac{1}{99}-\frac{1}{100}$

$=\frac{1}{3}-\frac{1}{100}=\frac{97}{300}$

`A=1/2+1/6+1/12+1/20+1/30+...+1/9900`

`=1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)+1/(5xx6)+...+1/(99xx100)`

`=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+...+1/99-1/100`

`=1/1-1/100`

`=100/100-1/100`

`=99/100`

$A=1/2+1/6+1/12+1/20+1/30+...+1/9900$

$=1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)+1/(5xx6)+...+1/(99xx100)$

$=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+...+1/99-1/100$

$=1/1-1/100$

$=100/100-1/100$

$=99/100$

Đáp án A

abab :  ab *5 +495-1000 = 101 *5 +495 -1000= 505 +495 - 1000 = 1000 - 1000 = 0

a) 1995 x 253 + 8910 : 495 = 504735 + 18 = 504753

b) 8700 : 25 : 4 = 8700 : (25 x 4) = 8700 : 100 = 87

ta có: 
1/2+1/6+...+1/9900 
=1/1.2+1/2.3...+1/99.100 
=1-1/2+1/2-1/3+1/3-...+1/99-1/100 
=1-1/100 
=99/100

\(A=\frac{1}{2}+\frac{1}{6}+\cdot\cdot\cdot+\frac{1}{9900}\)

\(A=\frac{1}{1\times2}+\frac{1}{2\times3}+\cdot\cdot\cdot+\frac{1}{99\times100}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\cdot\cdot\cdot+\frac{1}{99}-\frac{1}{100}\)

\(A=1-\frac{1}{100}\)

\(A=\frac{99}{100}\)

\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{9900}\)

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{99.100}\)

\(A=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{99}-\frac{1}{100}\)

\(A=\frac{1}{1}-\frac{1}{100}\)

\(A=\frac{99}{100}\)