The radius is 6cm.

Perimeter: l = pie r =18.84cm.

Area: S=2R Pie = = 37.68cm 2

2.

When the diameter of a circle is diagonal, the area of a square is the largest.

Diagonal: 8 decimeters = = 80 cm

According to Pythagorean theorem, x2+x2 = 80 2.

You can get: x = 40 * root number 2.

Square area: s = x * x = 3200 square centimeters.

3.

Actual: s = pair2 =112 * 3.14 = 379.94cm2.

After amplification: radius r = (22+4)/2 = 13.

After s = r 2 =13 2 * 31.4 = 530.66 cm 2.