sederhanakan (11/7-12/21)/(11/7+12/21)

= (11/7 - 12/21)/(11/7 + 12/21).
= (33/21 - 12/21)/(33/21 + 12/21).
= 19/21/45/21.
= 19/21 × 21/45.
= 399/945.
= 19/45.

gitu bukan? kalau salah, maaf ya