Question 181644
Pythagorean Theorem:  a^2 + b^2 = c^2

the diagonal is always longest side which is always c

(9)^2 + (3)^2 = c^2
81 + 9 = c^2
90 = c^2
sqr(90) = sqr (c^2)
sqr(9*10)= c
sqr(9) * sqr(10) = c
3sqr(10) = c

where sqr means square root