(1)因为m垂直于n,则m*n=0;
即sinB*(b*b-a*a-c*c)+(根号3*a*c)*cosB=0;
利用余弦定理:
a*a+c*c-b*b=2*a*c*cosB;
则sinB*cosB*2*a*c=根号(3*a*c)*cosB;
则sinB=根号3/2;
B=60度.
(2)根据余弦定理,代入B的值则a*a+c*c-b*b=a*c;
则a*a+c*c-9=a*c;而均值不等式则a*a+c*c>=2*a*c;
则a*c+9>=2*a*c;即a*c
(1)因为m垂直于n,则m*n=0;
即sinB*(b*b-a*a-c*c)+(根号3*a*c)*cosB=0;
利用余弦定理:
a*a+c*c-b*b=2*a*c*cosB;
则sinB*cosB*2*a*c=根号(3*a*c)*cosB;
则sinB=根号3/2;
B=60度.
(2)根据余弦定理,代入B的值则a*a+c*c-b*b=a*c;
则a*a+c*c-9=a*c;而均值不等式则a*a+c*c>=2*a*c;
则a*c+9>=2*a*c;即a*c