"Begin with two circles: C₁ of radius r₁ and C₂ which has half the radius, so 2r₂ = r₁."

(How Round Is Your Circle? Where Engineering and Mathematics Meet by J Bryant & C Sangwin, borrowed from Christchurch City Libraries)

#Page42
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Rules of the game:

Grab the nearest book.
Turn to page 42
Find the 2nd sentence
Post the sentence in a toot with the hashtag & write the rules as a comment to it
Don't look for your favourite, coolest or wittiest book. Go for the closest.