ac=2 bc=1 cos( c)=4/5 用余弦定理求ab:
ab^2=ac^2+bc^2-2*ac*bc*cos(c);
a+c+b=180度,sin(2a+c)=sin(a)cos(a+c)+sin(a+c)cos(a)
=-sin(a)cos(b)+sin(b)cos(a)
然后再用正弦定理余弦定理求值就可以了
ac=2 bc=1 cos( c)=4/5 用余弦定理求ab:
ab^2=ac^2+bc^2-2*ac*bc*cos(c);
a+c+b=180度,sin(2a+c)=sin(a)cos(a+c)+sin(a+c)cos(a)
=-sin(a)cos(b)+sin(b)cos(a)
然后再用正弦定理余弦定理求值就可以了