đáp án c

=\(\dfrac{5}{13}\).(\(\dfrac{8}{15}\)+\(\dfrac{26}{15}\)-\(\dfrac{8}{15}\))

=\(\dfrac{5}{13}\).\(\dfrac{26}{15}\)

=\(\dfrac{2}{3}\)

n≠31k+12

 có thể làm cách khác nhé

a)x-20/2(2/11.13+2/13.15+...+2/53.55)=3/11

   x-10(1/11-1/55)=3/11

   x-10.4/55=3/11

   x-40/55=3/11

   x=3/11+40/55

   x=1

1

gọi d là ƯCLN(18n+3,21n+7)

ta có 18n+3chia hết cho d

          21n+7 chia hết cho d

⇔21n+7-18n-3 chia hết cho d

⇔126n+42-126n-21 chia hết cho d 

21 chia hết cho d

⇒d∈Ư(21)=1;3;7;21

n ≠ 3k-1;3k-3;3k-7;3k-21

=1/3.(3/14.17+3/17.20+...+3/161.164)

=1/3.(1/14-1/17+1/17-1/20+...+1/161-1/164)

=1/3.(1/14-1/164)

=1/3.75/1148

=25/1148