sinA/a=sinB/b=sinC/c .
a/sinA=b/sinB=c/sinC=2R .
a=2RsinA，b=2RsinB，c=2RsinC .
Where r is the radius of the circumscribed circle of the triangle.
2. Cosine theorem.
cosA=(b？ +c？ -a？ ) /2bccosA= There are more adjacent sides than hypotenuse.
A 2 = B 2+C 2-2 in 200 BC.
B 2 = A 2+C 2-2 airlines.
C 2 = A 2+B 2-2 airlines.
cosC=(a^2+b^2-c^2)/(2 .
Cosb = (A 2+C 2-B 2)/(2, 200 BC.
Cosa = (c 2+b 2-a 2)/(2, BC 1000.
Test and analysis: From the nature of N(x), we can know that when x is odd, the largest odd factor of x is obviously itself. So N(x)=x, so we can decompose and calculate the s