Radius and chord properties

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also write down steps

Triangle ACD is congruent (identical to) Triangle BCD;
They both have the common side, CD, and have sides of common length, AC and CB as they are both radii;
As AC = CB, the angles CAB and CBA are equal also;
The fact triangles ACD and BCD are congruent means AD = DB;
We can use Pythagoras to find DB using CD and the radius:
DB² = r² - CD²
DB² = (51)² - (24)²
DB² = 2601 - 576
DB² = 2025
DB = 45mm
Since AD = DB, AB = 2(DB) = 2(AD) so:
AB = 2(45) = 90mm

Think of it as five or above give it a shove. change the 5 or above to the right to a zero and add one to the number on the left.