Question 287351
If the perimeter is 20cm, that means that each side must be 5cm (because the sides are equal in a square, and it has four sides: 5+5+5+5=20)

Use Pythagorean Theorem to find the diagonal:

{{{a^2+b^2=c^2}}}
{{{5^2+5^2=c^2}}}
{{{25+25=c^2}}}
{{{50=c^2}}}
{{{c=sqrt(50)}}}
c=7.07