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Ta có :

$B=\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+...........+\frac{1}{19}$

$B=\frac{1}{4}+\left(\frac{1}{5}+\frac{1}{6}+.......+\frac{1}{19}\right)$

Ta thấy :

$\frac{1}{5}>\frac{1}{20}$

$\frac{1}{6}>\frac{1}{20}$

..................

$\frac{1}{19}>\frac{1}{20}$

$\Rightarrow B>\frac{1}{4}+\left(\frac{1}{20}+\frac{1}{20}+.........+\frac{1}{20}\right)$($15$ p/s $\frac{1}{20}$

7/72

Rút gọn 4.7/9.32

Rút gọn 4.7/9.32

$A²=b.(a-c)-c.(a-b)$
$=a.b-b.c-a.c+b.c$
$=a.b+(-b.c+b.c)-a.c$
$=a.b+0-a.c$
$=a.b-a.c$
$=a.(b-c)$
 
Thay $a=-20$ ; $b-c=-5$ vào $a.(b-c)$ , ta có:
$-20.(-5)=100$

Ta có: $A²=100\Rightarrow A=\pm 10$ 

1/ (-145) - (18 - 145)

= -145 - 18 + 145

= -18

2/ 111 + (-11 + 27)

= 111 - 11 + 27

= 100 + 27

= 127

3/ (27 + 514) - (486 - 73)

= 27 + 514 - 486 + 73

= (27 + 73) + (514 - 486)

= 100 + 28

= 128

4/ (36 + 79) + (145 - 97 - 36)

= 36 + 79 + 145 - 97 - 36

= 79 + 145 - 97

= 127

5/ 10 - [12 - (-9) - 1]

= 10 - [12 + 9 - 1]

= 10 - 12 - 9 + 1

= 10 - 20

= -10

6/ (38 - 29 + 43) - (43 + 38)

= 38 - 29 + 43 - 43 - 38

= (38 - 38) + (43 - 43) - 29

= -29

7/ 271 - [(-43) + 271 - (-17)]

= 271 - [-43 + 271 + 17]

= 271 + 43 - 271 + 17

= 43 + 17

= 60

8/ -144 - [29 - (+144) - (+144)]

= -144 - [29 - 144 - 144]

= -144 - 29 + 144 + 144

= -29 + 144

= 115

g,x=3

a,x=4

B, x=3

c, x=1

d,X=64/3

 

kq :59