226. (1,2,3) , (1,2,4), (1,2,5), (2,3,4), (2,4,5), (3,4,5)
227. A(6,2), B(2m-1,2), C(7,2) (2m-1+ 6)/2 = 7 2m + 5 = 14 m = 11/2
228. AB = 6 AC = 8 BC = 12
se construieste un Δ oarecare ABC ; se construieste AD⊥BC;
in ΔADC AD² = AC² - CD²
in ΔADB AD² + AB² - BD² ⇒ AC² - CD² = AB² -BD²
AC² - AB² = (CD-BD)(CD+BD) 64 - 36 = 12(CD-BD)
CD-BD = 7/3
CD +BD = 12 2CD = 7/3+12 = 43/12 CD = 43/24
cosC = DC/BC = 43/(24·12) = 0,1493
