Radii are always positive. No, it is not possible to draw a circle with negative radius.
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1 centimeter
Yes
The diameter is a straight line from one side of the circle to the other which passes through the centre of the circle. The radius is a straight line from the centre to the boundary of the circle. To label them, you could either write the words alongside the lines or, if the circle is too small, draw arrows from the two lines to outside the circle and write the words at the end of the arrow.
Make a sketch of the situation. From a corner of the equilateral triangle draw a radius of the large circle, and from an adjacent side draw a radius of the smaller circle. You should have formed a small right-angled triangle with a known side of 10cm. and known angles of 30o, 60o and 90o. (The interior angles of an equilateral triangle are each 60o.) The hypotenuse is the unknown radius of the larger circle. But since cos 60 = 0.5, it is evident that the hypotenuse is 20cm. long.
It is possible. Just keep the drawing instrument 15.915 cm from a set point (the circle's center).