(y + \(\dfrac{1}{3}\)) + ( y + \(\dfrac{1}{9}\)) + ( y + \(\dfrac{1}{27}\)) + ( y + \(\dfrac{1}{81}\)) = \(\dfrac{56}{81}\)

( y + y + y + y ) + (\(\dfrac{1}{3}\)+ \(\dfrac{1}{9}\) + \(\dfrac{1}{27}\) + \(\dfrac{1}{81}\)) = \(\dfrac{56}{81}\)

4\(y\) + ( \(\dfrac{27}{81}\) + \(\dfrac{9}{81}\) + \(\dfrac{3}{27}\) + \(\dfrac{1}{81}\) ) = \(\dfrac{56}{81}\)

4y + \(\dfrac{40}{81}\) = \(\dfrac{56}{81}\)

4y = \(\dfrac{56}{81}\) - \(\dfrac{40}{81}\)

4y = \(\dfrac{16}{81}\)

y  = \(\dfrac{16}{81}\) : 4

y = \(\dfrac{4}{81}\)

\(\left(y+\dfrac{1}{3}\right)+\left(y+\dfrac{1}{9}\right)+\left(y+\dfrac{1}{27}\right)+\left(y+\dfrac{1}{81}\right)=\dfrac{56}{81}\)

\(\Rightarrow y+\dfrac{1}{3}+y+\dfrac{1}{9}+y+\dfrac{1}{27}+y+\dfrac{1}{81}=\dfrac{56}{81}\)

\(\Rightarrow4\times y+\dfrac{40}{81}=\dfrac{56}{81}\)

\(\Rightarrow4\times y=\dfrac{56}{81}-\dfrac{40}{81}\)

\(\Rightarrow4\times y=\dfrac{16}{81}\)

\(\Rightarrow y=\dfrac{16}{81}:4\)

\(\Rightarrow y=\dfrac{4}{81}\)

a, (-72).(15 - 49) + 15.(-56 + 72)

= -72 . 15 + 72.49  - 15.56 + 15.72

= (-72.15 + 15.72) + (72.49  - 15.56)

= 3528 - 84

= 2688

1b, 1532 + (-186) + (-1432) + (-14) + 123

= (1532 - 1432) - (186 + 14) + 123

= 100 - 200 + 123

= 223 - 200

= 23 

a) (x-3)(4-y)=-3

=> x-3 ; 4-y thuộc Ư(-3)={-1,-3,1,3}

Ta có bảng :

Vậy ta có các cặp x,y thõa mãn là : (2,7);(0,5);(4,1);(6,3)

b) S=1-2+3-4+...+99-100

S=(1-2)+(3-4)+...+(99-100)

S=(-1)+(-1)+...+(-1)

=>S=(-1).50

S=-50