Question

What is the length of the diagonal of square ABCD if AB = 7

Answer 1

bearing in mind that a square has four equal sides, and four right-angles, either diagonal will bisect the right-angle in two equal halves, leaving us with a 45-45-90 triangle.

check the picture below.

Answer 2

Use the pythagorean theorem.
a^2 + b^2 = c^2
so,
7^2 + 7^2 = diagonal
49 + 49 = 98
Square root of 98 is 9.899.....