Answer:[tex]d=28.28[/tex]Step-by-step explanation:To calculate the lenght of the diagonal d across the square, we can assume that the square it is compound of two right triangles. So, we can resolve this exercise using The Pythagorean Theorem.The Pythagorean theorem states that in every right triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the respective lengths of the legs. It is the best-known proposition among those that have their own name in mathematics.If in a right triangle there are legs of length a and b, and the measure of the hypotenuse is c, then the following relation is fulfilled: [tex]a^{2} +b^{2} =c^{2}[/tex] a is the height, b is the base, and c is the hypotenuse.To obtain the value of the hypotenuse [tex]c= \sqrt{a^{2} +b^{2} }[/tex]To find the value of the lenght of the diagonal d across the square, we have:[tex]d=\sqrt{a^{2} +b^{2} }[/tex] Where a = b = 20Substituting the values [tex]d=\sqrt{(20)^{2} +(20)^{2} }\\d=\sqrt{400+400} =\sqrt{800} \\d=28.284[/tex]Round the answer to 2 decimal places[tex]d=28.28[/tex]