a. (286 : 11 +7 x 2) x 3 - 13

= (26 + 14) x 3 - 13

= 40 x 3 - 13

= 120 - 13

= 107

b. 35 x 34 + 65 x 75 + 35 x 38 + 65 x 45 

= (35 x 34) + (65 x 75) + (35 x 38) + (65 x 45)

= 1190 + 4875 + 1330 + 2925

= (1190 + 1330) + (4875 + 2925)

= 2520 + 7800

= 10 320

a, 35.(34+86) +65.(75+45)= 35. 120 +65.120= 120.(35+65)=120.100=12000

a, 35x34+35x86+65x75+65x45

=35x(34+86)+65x(75+45)

=35x120+65x120

=120x(35+65)

=120x100

=12000

b,3x25x8+4x6x37+2x8x12

=24x25+24x37+24x8

=24x(25+37+8)

=24x70

=1680

35x34+35x38+65x45+65x27

=35x(34+38)+65x(45+27)

=35x72+65x72

=72x(35+65)

=72x100

=7200

35 x 34 + 35 x 38 + 65 x 45 + 65 x 27

= 35 x ( 34 + 38 ) + 65 x ( 45 + 27 )

= 35 x 72 + 65 x 72

= ( 35 + 65 ) x 72

= 100 x 72

= 7200

35 x 34 + 35 x 38 + 65 x 45 + 65 x 27

= 35 x (34 + 38) + 65 x ( 45 + 65)

= 35 x 72 + 65 x 110

= 2510 + 7150

= 9660

d) 35x34 +35x38 + 65x75 + 65x45

= 35 x ( 34 + 38 ) + 65 x ( 75 + 45 )

= 35 x 72 + 65 x 120

= 2520 + 7800

= 10 320

d) 35x34 +35x38 + 65x75 + 65x45

= 35 ( 34 + 38 ) + 65 ( 75 + 45)

= 35 * 72 + 65 * 120

= 2520 + 7800

= 10320

a) 12 x 53 + 53 x 172 - 53 x 84

= 53 x ( 12 + 172 - 84 )

= 53 x 100

= 5300

b) 35 x 34 + 35 x 95 + 65 x 75 + 65 x 54

= ( 35 x 34 + 35 x 95 ) + ( 65 x 75 + 65 x 54 )

= 35 x ( 34 + 95 ) + 65 x ( 75 + 54 )

= 35 x 129 + 65 x 129

= ( 35 + 65 ) x 129

= 100 x 129

= 12900

c) 86 x 7676 - 8686 x 76

= 86 x 76 x 101 - 86 x 101 x 76

= 0

d) 6969 x 45 - 4545 x 59

= 69 x 101 x 45 - 45 x 101 x 59

= 45 x 101 x ( 69 - 59 )

= 4545 x 10

= 45450

e) 351 x 67 + 351 x 16 + 649 x 83

= 351 x ( 67 + 16 ) + 649 x 83

= 351 x 83 + 649 x 83

= ( 351 + 649 ) x 83

= 1000 x 83

= 83000 

f) 42 x 53 + 47 x 156 - 47 x 114

= 42 x 53 + 47 x ( 156 - 114 )

= 42 x 53 + 47 x 42

= 42 x ( 53 + 47 )

= 42 x 100

= 4200

g) 3851 : 500 + 1149 : 500

= ( 3851 + 1149 ) : 500 

= 5000 : 500

= 10

h) 4783 : 300 - 1783 : 300

= ( 4783 - 1783 ) : 300

= 3000 : 300

= 10 

câu cuối hơi sai nên bạn gửi lại câu đó nha

có cần lời giải k cậu

chắc ko đâu

\(\dfrac{1}{3}+\dfrac{1}{15}+\dfrac{1}{35}+...+\dfrac{1}{323}\\ =\dfrac{1}{2}\times\left(\dfrac{2}{1\times3}+\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+...+\dfrac{2}{17\times19}\right)\\=\dfrac{1}{2}\times\left(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{17}-\dfrac{1}{19}\right)\\ =\dfrac{1}{2}\times\left(\dfrac{1}{1}-\dfrac{1}{19}\right)\\ =\dfrac{1}{2}\times\dfrac{18}{19}=\dfrac{9}{19}\)

đề bài đây: 

\(\dfrac{1}{3}+\dfrac{1}{15}+\dfrac{1}{35}+...+\dfrac{1}{323}\)