Answer:
√800 = 28 cm
Explanation:
Your c value is 40cm beacause c=length of the hypotenuse, or in this case the diagonal of the chess board.
You now have 2a2=402.
Start solving it:
2a2=402
2a2=1600
(2a2)/2=1600/2
a2=800
√(a2)=√800
a=√800 cm
You can plug this back in to check your answer, and also be sure that a=b is true.
a2+b2=c2
(√800)2+(√800)2=c2
800+800=c2
1600=c2
√1600=√c²
40=c