The diameter of a circle increases by 30 percent. What is the percentage increase in the area of the circle?

The diameter of a circle increases by 30 percent. What is the percentage increase in the area of the circle?

（1+30%）²-1=1.69-1=69%

The radius of a circle is 2 cm. What is the area of the circle?

Area 2x2x3.14 = 12.56 square centimeter
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Is the area of a circle 40 cm in diameter equal to the sum of the areas of two circles 20 cm in diameter? If not, who is bigger and how much?

Area (40 / 2) ^ 2 × 3.14 = 1256 square centimeter
Area 2 × (20 / 2) ^ 2 × 3.14 = 628 square centimeter
1256≠628
A large circle with a diameter of 40 cm