(5) (cotA)2+(cotB)2+(cotC)2+2(cotAcotB+cotBcotC+cotAcotC)>=3
(cotA+cotB+cotC)>=√3
(7) A+B+C=pi,cotAcotB+cotBcotC+cotAcotC=1
(cotA)2+(cotB)2+(cotC)2>=cotAcotB+cotBcotC+cotAcotC=1
(2) (cscA+cscB+cscC)(sinA+sinB+sinC)>=9
(cscA+cscB+cscC)>=9/(sinA+sinB+sinC)>=18/3√3=2√3
(3)A+B+C=pi,sin(C/2)=cos(pi/2-C/2)=cos[(A+B)/2]
cosA+cosB+cosC=2cos(A-B)/2*cos[(A+B)/2]+1-2[sin(C/2)]^2
=2cos(A-B)/2*sinC/2-sinC/2*cos[(A+B)/2]+1
=1+4sin(A/2)sin(B/2)sin(C/2)>1