how do you work out the radius of a circle?
917141
the radius is half the diameter.
409908
You have to find an area.  If ou have one foot by one foot, it is one square foot.  If the area is circular, find the area of the circle.
114473
If circumfrence = C (assume)\n   radius = r       (assume)\n   pi     = 3.14  or 22/7  (known value)\n\nthen :   C= 2 x (pi) x r \n          = 2 x  3.14 x 31/2\n          = 3.14 x 31   ( 2 gets cancelled out from the \n                          numerator and denominator)\n\n          = 97.34 units ( answer)
1095202
The equation of a circle is:\n(x-h)^2 + (y-k)^2 = r^2\nwhere:\nh: x-coordinate of the center\nk: y-coordinate of the center\nr: Radius of the circle\n\nThe center of the circle will be midway between the points. On the x-axis:\n\n(-4 + 8)/2 = 4/2 = 2\n\nOn the y-axis:\n\n(2+8)/2 = 10/2 = 5\n\nSo the location of the center as coordinates is (2,5)\nThe radius is simply the distance from the center to either point. So, using the pathegreom theorm and the center at (h,k) and the point at (Px,Py):\n\nsqrt((h-Px)^2 + (k-Py)^2) = r\n\nUsing (8,8) for the point and the center at (2,5):\n\nsqrt((2-8)^2 + (5-8)^2) = sqrt((-6)^2 + (-3)^2) = sqrt(36 + 9) = sqrt(45)\n\nObviously, you could plug square root of 45 into your calculator to get some decimal equivilent, but it's not necessary since the equation requires r^2 anyways. So our final equation, using the form above, is:\n\n(x-2)^2 + (y-5)^2 = (sqrt(45))^2\n\nSimplifying, the square root and ^2 cancel out leaving:\n\n(x-2)^2 + (y-5)^2 = 45
838513
Sigh - first guy had it right, but didn't explain it all.  \nPi = 3.14- all you need for the problem\nRadius= half the diameter of your circle. You find the midpoint and measure from there to the side of the circle, in any direction. Most problems give you the radius. or at least show you the midpoint on the circle.\n\nSo, it is 3.14 x the amount of the radius x 2= the area of the circle.\n\nYou can do this in reverse as well, if you know the area but not the radius.
858274
of a circle? you multiply 2 * pi * radius = circumference\nor\nmultiply 2 * diameter = circumference
303304
Use the well known formulas that relate the radius and diameter of a circle to its circumfrence.\n\nC = pi * D\nC = 2 * pi * R\n\nFor pi, you can use 3.14 or 22/7, or whatever value your teacher wants you to use.
695183
area of a circle = Пr²\ndiameter = 2.5\n\ntherefore radius = 1.25\n\ntherefore the area = 1.25² x П\n\narea = 4.9087...cm²
48612
Let r be the radius of the circle and s the length of the square's sides.\ncircle perimeter=2*pi*r, square perimeter=4*s\n2*pi*r=4*s => s=pi*r/2\ncircle area=pi*r^2, square area=s^2\ns^2=(pi*r/2)^2=pi^2*r^2/4\nBut pi<4 => pi^2*r^2/4<pi*r^2 => square area < circle area
492563
This cannot be the whole question.  Since you are using the letters r and d, I will assume you are talking about radius and diameter.\nAre you trying to find the area? then use the formula\nA = Pi*r^2\nIf you are trying to find circumference, then use\nC = 2*Pi*r\nRemember that d = 2r (diameter is twice the radius)
74729
The formula for the area of a circle is:\nPi*r^2/2\nto translate, "one half of Pi times the suare of the radius"\n\nSo, if you have a 3 cm radius:\n(3 cm)(3 cm) = 9 cm^2\n\nthen divide by two:\n9 cm^2 /2 = 4.5 cm^2\n\nthen multiply by Pi:\n\n4.5(Pi) cm^2\n\nIf you want a decimal number, you can use an estimation of Pi, something like 3.1415, but it's not exact since Pi is an irrational number (a non-ending and non-repeating decimal)
1167722
areah of a circle=pie times r squared  (r is radius)\ndiameter= 2r\nso r= d/2\n\nso you do 6sqrt(3)  divided by 2\n(you get 3 times sqrt of 3)\nthen do that number sqared....times pie(3.14)\n(18 times pie...more accurate if use calculaterbut answer is 18x3.14......)
593161
Simply use the corresponding equations. The circumference of a circle is Pi times the diameter and the area of a circle is Pi/4 times the diameter squared.\nThus doubling the diameter doubles the circumference, and quadruples the area.
52227
area of a circle is Пr2. hit the П key (shift -> exp (if you have a casio)).  the radius is 9.  square the radius. 9x9=81 (Пx81 or Пx9x9 = 254.4690049 <- that is your answer)
72316
The area of circle is pi*r^2 (pi r square). Now let the increased radius be R and iniitial radius be r.\nArea after expansion  =pi*R^2 (pi R square)\nArea before expansion =pi*r^2 (pi r square)\nIncrease in area      =pi*R^2  - pi*r^2\n                      =pi(R^2 - r^2) \n                      =pi(R+r)(R-r)\n                      =3*(7+5)(7-5), taking pi=3 aprox.\n                      =3x12x2\n                      =72 approx.\n Since answer c alone is near to this value the answer is C.\n\nThough the steps given are more the problem is done mentaly and not by writing these steps. The steps in mental calculations are \n1. 7+5=12\n2. 7-5=2\n3. 3x12x2=72 and choose the answer.
837293
I haven't done one of these in quite a while. \n\nWhat would you differential element be?  How about r*(dtheta)*dr?\n\nthat sort of a ds*dr   where ds depends on the radius.  Try it! \n\nNote that without the sin theta you'd get the int rdthetadr with r 0 to R and theta 0 to 2pi  You'd get the correct area of a circle.\n\nTry the intergral, see if it makes sense.  Then sketch the figure and estimate it's area to compare.  My confidence is increasing as I write this.
19668
The way to figure this out is, \n\nS=segment length of arc \nA=angle subtended by arc\nR is radius of circle \n\nS/A=2*pi*R/(2*pi),\n\nso S=RA\n\nthen differentiate with respect to time t, \n\nand dS/dt=R*dA/dt,\nand dA/dt=w, the angular speed\n\ndS/dt=T, tangential velocity\n\nso T=R*w