(1)题目有问题 sin^2乘2分之A=2c分之c-b才对
sin^2(a/2) = (1-cosa)/2 = (c-b)/2c
cosa = b/c = (b^2+c^2-a^2)/2bc
a^2+b^2 = c^2
直角三角形
（2）c=1,（a+b）^2