C = 6 / 15.18 + 6 / 18.21 + .......... + 6 / 87.90
3c= 3/15.18 + 3/18.21+...........+3/87.90
=1/15 - 1/18 + 1/18 -1/21 +.............+1/87-1/90
=1/15-1/90
=75/1350
C=75/1350 X 2
C=75/675 NHỚ TK CHO MÌNH NHA BẠN !
C=6 x \(\frac{1}{3}\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+.....+\frac{1}{87}-\frac{1}{90}\right)\)
C= 6 x \(\frac{1}{3}\left(\frac{1}{15}-\frac{1}{90}\right)\)
C= 6 x \(\frac{1}{3}x\frac{1}{18}\)
C =1/9
=6x(1/15x18+1/18x21+....1/87x90)
=6x1/15+1/90
=2/5+1/90
=37/90