(! ) find the radius of a circle so that its area and circumference have the same value.

the radius must equal 2

Step-by-step explanation:

A = pi * r^2       C = 2 * pi *r

set them equal

pi * r^2 = 2 * pi *r

divide both sides by pi

r^2 = 2 * r

divide both sides by r

r^2 /r = 2r/r

r = 2

the last choice

step-by-step explanation:

b. an equilateral triangle inscribed in a circle

step-by-step explanation:

we are given the figure of two circles having a triangle inside.

now, we know that,

"when a figure is lying inside an another figure, then the outside figure is known as circumscribed figure and the figure lying inside is referred to as the inscribed figure".

as we see that, the circles are containing the triangle completely.

thus, the circles are the circumscribed figures and the triangle is the inscribed figure.

hence, we get, 'an equilateral triangle is inscribed in a circle'.