The radius of the inscribed circle r=c/2.

2. Triangle area S = ABS Inc/2 = (A 2+B 2-C 2)/4.

sinC=(a^2+b^2-c^2)/2ab=cosC

tanC= 1

C=45 .

3.4=AB*BC*sinC/2

sinC=4/5

CosC=3/5 or -3/5.

4.a+b=3，∠C=60

c/sinC=2

C=2sinC= root number 3

cosC=(a^2+b^2-3)/2ab= 1/2

a^2+b^2=ab+3

a+b=3

(a+b)^2=3ab+3=9

ab=2

S=absinC/2= root number 3/2.