在△ABC中,角A,B,C的对边分别为a,b,c,且bcosC=3acosB-ccosB. (Ⅰ)求cosB的值; (Ⅱ)若BA•BC=2,且b=22,求a和c的值.
问题描述:
在△ABC中,角A,B,C的对边分别为a,b,c,且bcosC=3acosB-ccosB.
(Ⅰ)求cosB的值;
(Ⅱ)若
•BA
=2,且b=2BC
,求a和c的值.
2
答
(I)由正弦定理得a=2RsinA,b=2RsinB,c=2RsinC,则2RsinBcosC=6RsinAcosB-2RsinCcosB,故sinBcosC=3sinAcosB-sinCcosB,可得sinBcosC+sinCcosB=3sinAcosB,即sin(B+C)=3sinAcosB,可得sinA=3sinAcosB.又sinA≠0...