What does C mean in the area of a circle?

Perimeter

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1. 5 out of 8 meters represents a fraction of 1 meter and a fraction of several meters
2. How many grams per square meter is an ounce per square yard
3. Number reasoning questions 1,3,4,6,11,19, ()
4. Any function f (x) defined on R can be expressed as the sum of an odd function g (x) and an even function H (x), X belongs to R, then, A,g（x）=x,h（x）=lg(10^x+10^-x+2) B,g(x)=1/2lg(10^x+1+x),h(x)=1/2lg(10^x+1+x) c,g(x)=1/2x,h(x)=lg(10^x+1)-x/2 D,g(x)=-1/2x,h(x)=lg(10^x+1)+x/2 Why not D
5. A square with a side length of 4 decimeters, take one side of it as the axis, and rotate the square for one circle to get a square______ What is the volume of this body______ Cubic decimeter
6. The formula of mathematics practical problems in grade six
7. 1+2-3-4+5+6-7-8+9+10-11-12······-2003-2004+2005+2006-2007-2008+2009+2010=?
8. I go to the library every day
9. When x=______ F (x) = (x-a1) 2 + (x-a2) 2 + +(x-am) 2 is the minimum
10. My mother said to Xiao Ming, "I'll go to the unit for a meeting this afternoon, and I won't pick you up after school."?
11. 2 / 7,3 / 11,1 / 3,7 / 19
12. Let the density function of random variable X be symmetric with respect to x = u, and prove that its distribution function satisfies the following requirements: F (U + x) + F (u-x) = 1 (x is between positive and negative infinity)
13. Three fifths of a number is nine fifths of a number. What's five sixths of a number?
14. What is the general formula of 1,1,2,3,4,5,6,7,8,9,10?
15. Number reasoning 8,11,4,5,21,20,11 () Civil servants, why
16. If x = 1 and 1 / 5 minus 9 / 20, y = 1 and 9 / 5 times 6 / 7, then are x and Y reciprocal
17. If all the vertices of a cuboid are on the same sphere and the lengths of the three edges on one vertex are 1, 2 and 3 respectively, the surface area of the cuboid is 0______ ．
18. In the parallelogram ABCD, we know AB = 10, root 3, B = 60 ° and AC = 30, then we can find the area of the parallelogram Speed, speed! Thank you! ~
19. There are four points P, a, B, C on the sphere, PA, Pb, PC are perpendicular, PA = 3, Pb = 4, PC = 5, so how to calculate the sphere area?
20. As shown in the figure, there are four points P, a, B and C on the sphere. If PA, Pb and PC are perpendicular to each other, and PA = Pb = PC = a, the surface area of the sphere is______ ．