2/5 + 3/4 = 23/40

7/12 - 2/7 + 1/12 =  25/84 + 1/12 = 8/21

12/17 - 5/17 - 4/17 =  3/17

OK

\(\frac{2}{5}+\frac{3}{4}=\frac{8}{20}+\frac{15}{20}=\frac{23}{20}\)

\(\frac{7}{12}-\frac{2}{7}+\frac{1}{12}=\left(\frac{7}{12}+\frac{1}{12}\right)-\frac{2}{7}=\frac{2}{3}-\frac{2}{7}=\frac{14}{21}-\frac{6}{21}=\frac{8}{21}\)

\(\frac{12}{17}-\frac{5}{17}-\frac{4}{17}=\frac{12-5-4}{17}=\frac{3}{17}\)

trần thanh hải sai con đầu bằng 23/20 mới đúng

a) 2/5+3/4=23/20

7/12-2/7+1/12=8//21

 12/17-5/17-4/17=3/17

b) 675,39+342,14=1017,53

563,87+403,13-328,35=638,65

a)            2/5+3/4

           =8/20+15/20

           =21/20

             7/12-2/7+1/12

            =7/12+1/12-2/7

             =1/4-2/7

              =7/28-8/28

              =-1/28

                 12/17-5/17-4/17

               =12/17-9/17

               =3/17

b)              675,39+342,14

               =       1017,53

                   563,87+403,13-328,35

                =       967-328,35

                 =  638,65

Bài 1: 

a: =25+75=100

b: =60-17-43+12=12

c: =-2-18=-20

d: =-3+36-17=36-20=16

Bài 2: 

a: =-102

b: =-1000

c: =12x15=180

d: =21x(-10)=-210

B1

a 100

b 12

c -20

d 16

B2

a -102

b -1000

c 180

d -210

Bài 1:Tính

 a)(-12)+25+75+12 = 100

b)60+12+(-17)+(-43)=12
c)(-2)+(-87)+(-18)+87=-20
d)(-1)+(-2)+36+(-17)=16
Bài 2:Tính
b)8 . (-125)=-100
c)(-12) . (-15)=180
d)21 . (- 3)+(- 21 ) . 7=-210

phép tính cứ làm bình thường nha nhân chia trước cộng trừ sau , cái gì trung đặt ra ngoài .xin lỗi vì vt hơ mỏi tay nên mik vt lời thôi nha 

~ hok tốt ~

cút m*** đi!!

Đặt \(A=\sqrt{17+12\sqrt{2}}+\sqrt{17-12\sqrt{2}}\)

\(\Rightarrow A^2=17+12\sqrt{2}+17-12\sqrt{2}+2\sqrt{\left(17+12\sqrt{2}\right)\left(17-12\sqrt{2}\right)}\)

\(\Rightarrow A^2=34+2\sqrt{289-288}\)

\(\Rightarrow A^2=34+2=36\)

\(\Rightarrow A=\sqrt{36}=6\)

                                          Vậy \(\sqrt{17+12\sqrt{2}}+\sqrt{17-12\sqrt{2}}=6\)

\(\sqrt{17+12\sqrt{2}}-\sqrt{17-12\sqrt{2}}\)

\(=\sqrt{3^2+2\cdot3\cdot2\sqrt{2}+\left(2\sqrt{2}\right)^2}-\sqrt{3^2-2\cdot3\cdot2\sqrt{2}+\left(2\sqrt{2}\right)^2}\)

\(=\sqrt{\left(3+2\sqrt{2}\right)^2}-\sqrt{\left(3-2\sqrt{2}\right)^2}\)

\(=\left|3+2\sqrt{2}\right|-\left|3-2\sqrt{2}\right|\)

\(=3+2\sqrt{2}-3+2\sqrt{2}\)

\(=4\sqrt{2}\)

\(\sqrt{17+12\sqrt{2}}-\sqrt{17-12\sqrt{2}}\)

\(=\sqrt{3^2+2.3.2\sqrt{2}+\left(2\sqrt{2}\right)^2}-\sqrt{3^2-2.3.2\sqrt{2}+\left(2\sqrt{2}\right)^2}\)

\(=\sqrt{\left(3+2\sqrt{2}\right)^2}-\sqrt{\left(3-2\sqrt{2}\right)^2}\)

\(=\left|3+2\sqrt{2}\right|-\left|3-2\sqrt{2}\right|=\left(3+2\sqrt{2}\right)-\left(3-2\sqrt{2}\right)\)

\(=3+2\sqrt{2}-3+2\sqrt{2}=4\sqrt[]{2}\)

42/84

3/17 tk mk nhé mk nhanh nhất tk giùm đi

8/21

3/17

\(\frac{7}{12}-\frac{2}{7}+\frac{1}{12}=\frac{8}{21}\)                                                                                                                                                      \(\frac{12}{17}-\frac{5}{17}-\frac{4}{17}=\frac{3}{17}\)

\(\left(\frac{3}{17}-\frac{2}{345}+\frac{5}{12}\right)-\left(-\frac{2}{345}+\frac{3}{17}-\frac{1}{12}\right)\)

\(=\frac{3}{17}-\frac{2}{345}+\frac{5}{12}+\frac{2}{345}-\frac{3}{17}+\frac{1}{12}\)

\(=\left(\frac{3}{17}-\frac{3}{17}\right)+\left(\frac{2}{345}-\frac{2}{345}\right)+\frac{5}{12}+\frac{1}{12}\)

\(=\frac{6}{12}=\frac{1}{2}\)

( 3/17 - 2/345 + 5/12 ) - ( - 2/345 + 3/17 - 1/12 )

= 3/17 - 2/345 + 5/12 + 2/345 - 3/17 + 1/12

= ( 3/17 - 3/17 ) + ( 2/345 - 2/345 ) + ( 5/12 + 1/12 )

= 0 + 0 + 1/2

=1/2

a) (-12) + 25 + 75 + 12 = (-12) + 12 + 25 + 75 = 0 +100 = 100.

b) 60 +12 + (-17) + (-43) = 60 + [(-47) + (-43)] +12.

c) (-2) + (-87) + (-18) + 87 = [(-2) + (-18)] + [(-87) + 87].

d) (-1)+(-2) + 36 + (-17) = 36+[(-1) + (-2) + (-17)].

a, (-12) + 25 + 75 + 12

= [ (-12) + 12] + (25 + 75)

= 0 + 100

= 100

b, 60 + 12 + (-17) + (-43)

= -(17 + 43) + 60 + 12

= - 60 + 60  + 12

= 0 + 12

= 12