用余弦定理：cosC=(a^2+b^2-c^2)/2ab.
sinC=√(1-cos^2C)=√{1-[(a^2+b^2-c^2)/2ab]^2}
tanC=sinC/cosC=√{1-[(a^2+b^2-c^2)/2ab]^2/[(a^2+b^2-c^2)/2ab].