JAMB 1987 · UME · Q42

What is the circumference of the circle of latitude $\theta^\circ$ on the earth, whose radius is $R$ ?

Worked solution (try it first)

The circle of latitude $\theta$ has its centre on the earth's axis, and its radius is $r = R\cos\theta$ (from the right-angled triangle with hypotenuse $R$ ).
Its circumference is $2\pi r = 2\pi R\cos\theta$ , option B.