Question 703200
Description is a half of a square, along the diagonal.  If the side of the original square is 5, then the diagonal (here the distance from A to B).
If a is 5 then b is also 5 because this is isosceles. AC and BC are equal.


{{{drawing(200,200,0,6,0,6,
   line(1,1,1,5),
   line(1,5,5,1),
   line(5,1,1,1),
  locate(1,5.5,A),
  locate(1,.5,C),
  locate(0.05,2.8,b=5),
  locate(5.5,1,B)
)}}}


If a=5,
{{{5^2+5^2=c^2}}}
{{{50=c^2}}}
{{{2*5*5=c^2,}}}
{{{c=5*sqrt(2)}}}