Consider six squares with sides of different lengths. The length of side of square is in direct proportion to:
Perimeter of the square
Both of these
Length of squarePerimeter of square=a4a=14
which is a constant.
Length of squareArea of square=aa2=1a
which is not constant.
Length of squareLength of diagonal=1√2
which is constant.
Hence, length of a square is in direct proportion with its perimeter and diagonal.