notam a=BC
in Δ NDC dr in D, avem ctg C=CD/ND= a/2 /(DM+MN)= a/2 / 12=a/24
in Δ MDB dr in D, avem ctg B=BD/MD= a/2 /3= a/6
dar m(B+C)= 90 ⇒ ctg (B+C)= 0
ctg (B+C)= (ctg B* ctg C -1)/ (ctg B+ctg C)
0= (a/6*a/24 -1)/ (a/6+ a/24)= [(a²- 144)/24*6] * 24/5a= (a²-144)/ 30a
⇒ a²- 144= 0
⇒ a=12
Deci BC=12
O zi buna!