book Maths Puzzle Numerical Ability

I am unable to solve this problem. Kindly help.

Log10(x+y)/3 = 1/2(log10x + log10y) Now, prove that x/y + y/x = 7

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book Other Question

How would you know where to put the line on the graph? If i was to graph this system of linear inequalities, how will it look on a graph? y≥x and y≥-x/2+1?