... Tacú: v theo(8)cú (11) Tngttacú: ư13 (12) Thay(11),(12)vo(1) cú: ( ) (13) T(9)v(13)cú (14) T(3)(4) v(14)suyra Haysaukhithay Tacú : (15) Thay(7)vo(15)cú: Hay (*) Vy(*)ỳng vliucnchngminh. Bi3:(nhlớhmscosthnhtvitgiỏc) ... toỏn hay v rt thỳv n nhng cụng thc khụ khan v quỏ tru tng.Bnsthymtsbitoỏnmcúthchngdựnglmgỡc,nhnghóycoinúnhmt bụnghng,mtbchophaymtbntỡnhcacathiờnngtoỏnhcVỡmtl,núcsinh ratrongmtphỳt,mtngy,cmtingihayquanhiuthhchthomónnimammờ ... 2) Vi e = AC , f =BD ,thay(2)vo(1): KL2 = a + b + c + d - e - f = a + b + ac cos ( b +g )(3) Liỏpdngnhlớhmscos,tacú: e = a + b - abcos b (4) f = c + b -2bc cos g (5) Thay(4)v(5)vo(3),tacú: d...