1a,=-2*3=-6                      b,=2*3=6

2a,=-39         b,=-12

B1 :

a, = -2-2-2= -6

b, = -2-2-2 = -6

B2:a, = 768-39-768 = (768-768)-39=0-39=-39

b, = -1579-12+1579 = (-1579+1579)-12 = 0-12=-12

k mk nha

Bài 1 : Tính : 

a, 1+(-3)+5+(-7)+9+(-11)

= ( -2 ) + 5 + ( -7 ) + 9 + ( -11 )

= 3 + ( -7 ) + 9 + ( -11 )

= ( -4 ) + 9 + ( -11 )

= 5 + ( -11 )

= ( -6 )

b, (-2)+4+(-6)+8+(-10)+12.

= 2 + ( -6 ) + 8 + ( -10 ) + 12

= ( -4 ) + 8 + ( -10 ) + 12

= 4 + ( -10 ) + 12

= 6 + 12

= 18

Bài 2 : Tính nhanh :

a, (768-39)-768

= 768 - 39 - 768

= ( 768 - 768 ) - 39

= 0 - 39

= ( -39 ) 

b, (-1579)-(12-1579).

= ( -1579 ) - 12 + 1579

= [ ( -1579 ) + 1579 ) ] - 12

=  0 - 12

= ( -12 )

D=2/3(1+1/2+...+1/256)

=2/3*511/512=511/768

=\(2.\left(\frac{1}{3}+\frac{1}{6}+\frac{1}{12}+\frac{1}{24}+\frac{1}{48}+\frac{1}{98}+\frac{1}{192}\right)\)

=2.\(\frac{6272+3136+1568+784+392+192+98}{18816}\)

=24884/18816

=1,32

đây không phải là tính nhanh

\(2\times169\times12-3\times68\times8-24\)

\(=24\times169-24\times68-24\)

\(=24\times\left(169-68-1\right)\)

\(=24\times100=2400\)

Đặt \(A=\frac{2}{3}+\frac{2}{6}+\frac{2}{12}+\frac{2}{24}+\frac{2}{48}+\frac{2}{96}+\frac{2}{192}\)

\(\frac{1}{2}A=\frac{1}{3}+\frac{1}{6}+\frac{1}{12}+\frac{1}{24}+\frac{1}{48}+\frac{1}{96}+\frac{1}{192}\)

\(\frac{1}{4}A=\frac{1}{6}+\frac{1}{12}+\frac{1}{24}+\frac{1}{48}+\frac{1}{96}+\frac{1}{192}+\frac{1}{384}\)

\(\frac{1}{2}A-\frac{1}{4}A=\frac{1}{3}-\frac{1}{384}\)

\(\frac{1}{4}A=\frac{127}{384}\)

\(A=\frac{127}{384}.4=\frac{127}{96}\)

khong ro moi hoi

2/3+2/6+2/12+2/24+2/48+2/96+2/192

=2/3+1/3+1/6+1/12+1/24+1/48+1/96

=\(\frac{64+32+16+8+4+2+1}{96}=\frac{127}{96}\)

Dễ lắm làm như này nè :I don't know

127/96 nhé bạn