What is the radius of the following circle which center is O, given that:
AB = 3x - 3 and OA = x + 3.
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The circles in the picture are externally tangent. The distance between the centers is 28 cm and the difference between their radii is 8 cm. Find the length of each radius.
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Knowing that O is the center of the circle, find x in the following cases:
a) b)
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AB is the diameter of the circle whose center is O and the triangle ABC is inscribed in. The quotient where the area of the triangle ACO is divided by the area of the triangle COB is:

a) b) c)
d) e)

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(FESP - 1991) An equilateral triangle ABC is inscriibed in a circle whose radius is 6 cm length. The triangle is intercepted by a diameter MN of the circle, forming a trapezoid, as shown in the picture below. We can say that the quotient of the triangle ABC area divided by the trapezoid APQC area is:

a) b) c)
d) e)

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In the following picture: is the diameter of the sphere whose center is , é is the center of a intersection with a plane perpendicular to . Also the measures are and . Calculate the area of the cicle whose center is .
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