Given
#x+ypropz#
#=>x+y=mz.......[1]#, where m = proportionality constant
#=>(x+y)/z=m#
#=>(x+y+z)/z=m+1 ....[2]#
Again
#y+zpropx#
#=>y+z=nx........[3]#, where n = proportionality constant
#=>(y+z)/x=n#
#=>(x+y+z)/x=n+1 ......[4]#
Dividing [2] by [4]
#x/z=(m+1)/(n+1)=k(say)#
#=>x=kz......[5]#
By [1] and [5] we get
#kz+y=mz#
#=>y=(m-k)z#
#=>y/z=(m-k)......[6]#
Dividing [2] by [6] we get
#(x+y+z)/y=(m+1)/(m-k)=c " another constant"#
#=>(x+y+z)/y-1=c -1#
#=>(x+z)/y=c -1="constant"#
Hence
#z+xpropy#
Proved