Answer:
7√2 ≈ 9.9 dm
Explanation:
You want the radius of a circle when tangents from a point 14 dm from the center make a right angle.
Square
The attached figure shows all of the angles between radii and tangents are right angles. Effectively, the tangents and radii make a square whose side length is the radius of the circle. The diagonal of the square is given as 14 dm. We know this is √2 times the side length, so the length of the radius is ...
r = (14 dm)/√2 = 7√2 dm ≈ 9.8995 dm ≈ 9.90 dm
The radius is about 9.90 dm.
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Additional comment
The angles at A and O are supplementary, so both are 90°. The angles at the points of tangency are 90°, so the figure is at least a rectangle. Since adjacent sides (the radii, the tangents) are congruent, the rectangle must be a square. The given length is the diagonal of that square.
For side lengths s, the Pythagorean theorem tells you the diagonal length d satisfies ...
d² = s² +s² = 2s²
d = s√2
d/√2 = s . . . . . . . . the relation we used above
This relationship between the sides and diagonal of a square is used a lot, so is worthwhile to remember.