... === Ccosz;Bcosy;Acosx)2;0(C;B;A:ABC10b) Nếu =++>xyzzyxz;y;x 0 thì === tgCz;tgBy;tgAx)2;0(C;B;A:ABCc) Nếu =++>1zxyzxy0z,y;x thì === === 2Ctgz;2Btgy;2Atgx);0(C;B;AgCcotz;gBcoty;gAcotx)2;0(C;B;A:ABC2. ... === tgCz;tgBy;tgAx)2;0(C;B;A:ABCc) Nếu =++>1zxyzxy0z,y;x thì === === 2Ctgz;2Btgy;2Atgx);0(C;B;AgCcotz;gBcoty;gAcotx)2;0(C;B;A:ABC2. Các ví dụ minh ... +=+=+===cossin32cossinAcos2bsin1acos2bsin1a22A 2)62sin(22cos212sin2322cos2sin3 === (đpcm)VD4: Cho a, b thoả mãn : 7 12b 5a++= 13Chứng minh rằng: a2 + b2 + 2(b-a) -...