(1) Through calculation, in ①, the minimum diameter of the disk just covering the square cardboard should be 510cm; ;

(2) The minimum diameter of the disc needed to cover the three square cardboard in Figure ② is 102cm.

Fig. ③ The minimum diameter of the disc required to cover three square cardboard is102 cm; ;

(3) If the distance from the center of the circle to the top horizontal line is X and the distance from the bottom horizontal line is Y, then x+y= 10,

∵OB=OA,∴AK=BK，

According to Pythagorean theorem,

x2+(52)2=y2+52，

X= 19×5 16，y= 13×5 16，

Then ob = (13× 516) 2+52 ≈ 6.44,

The minimum diameter of the disc is 12.88 cm. ..

(4) From the above conclusion, we can draw a conclusion that a. A disk can cover three square cardboard.