13. a) Since AC and AB are tangents to the circle, we know that two tangents that meet at an external point are equal.
Hence, AB = AC; so, AB = 40 ft.
13. b) (20 + r)² = 40² + r²
400 + 40r + r² = 1600 + r²
40r = 1200
r = 30
Hence, the diameter is 60 ft.
