Givea2,b2,c2areinAP
b2−a2=c2−b2
k2sin2B−k2sin2A=k2sin2C−k2sin2B
sin(B+A)sin(B−A)=sin(C+B)sin(C−B)
sin(π−C)sin(B−A)=sin(π−A)sin(C−B)
−sinCsin(B−A)=−sinA−sin(C−B)
sinBcosA−cosBsinAsinA=sinCcosB−cosCsinBsinC
sinBcotA−cosB=cosB−cotCsinB
cotA−cotB=cotB−cotC
cotA,cotB,cotCareinAP