Answer:
AT = √(a² - r²)
Explanation:
Pythagorean Theorem (NOTE: the hypotenuse is ALWAYS c!!!):
1. Let AT = t... Then, t² + r² = a²
2. Solve for t: t² = a² - r²
√(t²) = √(a²-r²) [we'll only use the principal root, since we're dealing with length]
3. Finalize: t = AT = √(a² - r²)