Question 879943
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Hi,
use The Pythagorean theorem...
20^2 + 20^2 = c^2
  800 = c^2
 {{{sqrt(800)}}} = c   |Tossing out negative solution for unit measure
 {{{sqrt(100*4*2)}}} = c
    20&#8730;2 = c, the diagonal
Note: pattern for an Isosceles right triangle