2 bài tập của chiếm:

1 - ( 2/3 + 20%) = 2/15 ( tổng số bài)

Trong 3 ngày An làm được:

2 : 2/15 = 15 ( bài )

Đ/s: 15 bài

1 + 2 + 3 + ... + 199 + 200

Ta có : 1 + 2 + 3 + ... + 199 + 200 ( có 200 số )

      = (200 + 1) x 200 : 2 = 20100

3 + 5 + 7 + ... + 97 + 99

Ta có : 3 + 5 + 7 + ... + 97 + 99 (có 49 số )

      = (99 + 3) . 49 : 2 = 2499

A = 1 + 2 + 3 + ... + 199 + 20

A = ( 199 + 1 ) . 199 : 2 +20 = 19920

B = 3 + 5 + 7 +... +  97 + 99

có số số B là : ( 99 - 3 ) :2 + 1 = 49

B = ( 99 + 3 ) . 49 : 2 = 2499

có số số C là : ( 98 - 2 ) : 2 + 1 = 49

C = ( 98 + 2 ) . 49 : 2 = 2450

2 . lxl=20

lxl=20 : 2

lxl=10

<=>x= -10

hoặc x= 10

=>1/2x+3/5x-6/5=3

=>11/10x=21/5

=>x=42/11

\(A=\frac{1\cdot2+2\cdot4+3\cdot6+4\cdot8+5\cdot10}{3\cdot4+6\cdot8+9\cdot12+12\cdot16+15\cdot20}\)

\(=>A=\frac{1\cdot2+4\cdot1\cdot2+9\cdot1\cdot2+16\cdot1\cdot2+25\cdot1\cdot2}{3\cdot4+4\cdot3\cdot4+9\cdot3\cdot4+16\cdot3\cdot4+25\cdot3\cdot4}\)

\(=>A=\frac{\left(1+4+9+16+25\right)\cdot1\cdot2}{\left(1+4+9+16+25\right)\cdot3\cdot4}=\frac{1}{6}=\frac{111111}{666666}\)

Mà \(\frac{111111}{666666}< \frac{111111}{666665}\)

\(=>A< B\)

Ta có:

\(B=2\cdot\left(1\cdot99+2\cdot98+...+50\cdot50\right)-50\cdot50\)

\(=2\cdot\left(1\cdot99+2\cdot\left(99-1\right)+...+50\cdot\left(99-49\right)\right)-50\cdot50\)-

\(=2\cdot\left(1\cdot99+2\cdot99-1\cdot2+...+50\cdot99-49\cdot50\right)-50\cdot50\)

\(=2\cdot\left(\left(1\cdot99+2\cdot99+...+50\cdot99\right)-\left(1\cdot2+2\cdot3+...+49\cdot50\right)\right)-50\cdot50\)

\(=2\cdot\left(\frac{99\cdot50\cdot51}{2}-\frac{49\cdot50\cdot51}{3}\right)-50\cdot50\)

\(=2\cdot84575-2500\)

\(=166650\)

Vậy B=166650

A=1.99+2.98+3.97+...+97.3+98.2+99.1
A=1.99+2.(99−1)+3.(99−2)+...+98.(99−97)+99.(99−98)
A=1.99+2.99−1.2+3.99−2.3+98.99−97.98+99.99−98.99
=(1.99+2.99+3.99+...+98.99+99.99)−(1.2+2.3+3.4+...+97.98+98.99)
=99.(1+2+3+...+98+99)−(1.2+2.3+3.4+...+97.98+98.99)
=99.4950−(1.2+2.3+3.4+97.98+98.99)
Mà 1.2+2.3+3.4+...97.98+98.99
=​ 1/3 ​.[1.2+2.3.(4−1)+3.4.(5−2)+98.99.(100−97)]
=1/3​​.98.99.100

=323400
⇒A=99.4950−323400=166650

A= -5/1/7+32/5=0.22857142857142286

B= -2/1/3-1/2/7=119047619047619

A = 1 . 2  + 2 . 3 + 3 . 4 + 4 . 5 + ... + 98 . 99

3A = 1 . 2 . 3 + 2 . 3 . 3 + 3 . 4 . 3 + 4 . 5 . 3 + ... + 98 . 99 . 3

3A = 1 . 2 . 3 + 2 . 3 . ( 4 - 1 ) + 3 . 4 . ( 5 - 2 ) + 4 . 5 . ( 6 - 3 ) + ... + 98 . 99 . ( 100 - 97 )

3A = 1 . 2 . 3 + 2 . 3 . 4 - 1 . 2 . 3 + 3 . 4 . 5 - 2 . 3 . 4 + 4 . 5 . 6 - 3 . 4 . 5 + ... + 98 . 99 . 100 - 97 . 98 . 99

3A = 98 . 99 . 100

A = \(\frac{98.99.100}{3}\)

A = 323400

dễ lắm 

chờ 5 phút nhé 

để có việc

Ta có:

1/2^2+1/3^2+.....+1/20^2>1/2.2+1/3.4+1/4.5+.....+1/20.21

                                     =1/4+1/3-1/21

                                      =1/4+6/21

                                      =45/84>1/2

Ta có:

1/2^2+1/3^2+..........+1/20^2<1/1.2+1/2.3+.....+1/19.20

                                           =1-1/20

                                           =19/20<1

A = 1 - 1/20

= 19/20

Thử: 1/2 < 19/20 < 1

Đs: 19/20