Perimeter of the triangle

Geometry Level 3

The figure above shows a circle with A A as its center and has a radius of 5.

E C \overline{EC} and E F \overline{EF} are both tangent to the circle, and D G \overline{DG} is a secant line that passes through A A and intersects with the circle at G G .

E D B = 9 0 \angle EDB = 90 ^ \circ , and D B = 4 \overline{DB} = 4 .

Find the perimeter of D B A \triangle DBA .


The answer is 16.

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1 solution

Valentino Wu
Jan 30, 2019

Connect A C \overline{AC} and draw a perpendicular line from B B down to A C \overline{AC} and let the intersection be point I I . Because both E D B \angle EDB and D C A \angle DCA are 9 0 90 ^ \circ , B I \overline{BI} would be parallel and equal to D C \overline{DC} , C I \overline{CI} would also be perpendicular and equal to D B \overline{DB} , therefore A I = 5 4 = 1 \overline{AI}\ = 5 - 4 = 1 , and B I A = 9 0 \angle BIA = 90 ^ \circ .

According to the Pythagorean theorem:

D C \overline{DC} = B I \overline{BI} = 5 2 1 2 = 2 4 \sqrt{5^2 - 1^2} = \sqrt 24 .

Then, according to the Tangent-secant theorem:

D C 2 = D H × D G \overline{DC}^2 = \overline{DH}\ \times \overline{DG} .

Let D H = x \overline{DH} = x

x ( 10 + x ) = 24 x(10 + x) = 24

x 2 + 10 x 24 = 0 x^2 + 10x - 24 = 0

( x + 12 ) ( x 2 ) = 0 (x+12)(x-2)=0

x = 12 x = -12 or 2 2 , 2 2 being the only valid answer here.

Therefore, P D B A = 2 + 5 + 4 + 5 = 16 P_{\triangle DBA} = 2 + 5 + 4 + 5 = 16

Cute problem

Valentin Duringer - 1 year ago

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