1. The formula for circumference is 2πr. From that formula you can see that anything you do to the radius (in terms of multiplying), you do to the circumference in the same way. Double the radius, you double the circumference, triple the radius, you triple the circumference. This is because the 'r' is in the formula without any exponent.
2. The formula for the area is πr². This is different. Now if you double the radius (multiply it by 2), the area is multiplied by 2², so by 4. The power of 2 in the formula works on the multiplication factor you apply. If you make the radius p times as big, the area becomes p² times as big.
