x²+y²=25xy
adding 2xy on both sides
x²+y²+2xy=27xy
[x+y]²=27xy
taking log on both sides
log[x+y]²=log27xy
log(abc)= log a+ log b+ log c
Log27xy=log(27×x×y)
=Log(3³×x×y)
=Log 3³ + log x+ log y
Log[x+y]²= 2 log [x+y]
Now,
Log[x+y]²= log 27xy
2log[x+y]=log3³+log x+log y
2 log[x+y]=3log3+logx+logy