The figure above shows a circle with as its center and has a radius of 5.
and are both tangent to the circle, and is a secant line that passes through and intersects with the circle at .
, and .
Find the perimeter of .
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Connect A C and draw a perpendicular line from B down to A C and let the intersection be point I . Because both ∠ E D B and ∠ D C A are 9 0 ∘ , B I would be parallel and equal to D C , C I would also be perpendicular and equal to D B , therefore A I = 5 − 4 = 1 , and ∠ B I A = 9 0 ∘ .
According to the Pythagorean theorem:
D C = B I = 5 2 − 1 2 = 2 4 .
Then, according to the Tangent-secant theorem:
D C 2 = D H × D G .
Let D H = x
x ( 1 0 + x ) = 2 4
x 2 + 1 0 x − 2 4 = 0
( x + 1 2 ) ( x − 2 ) = 0
x = − 1 2 or 2 , 2 being the only valid answer here.
Therefore, P △ D B A = 2 + 5 + 4 + 5 = 1 6