In right △ A B C , m ∠ A B C = 3 0 ∘ , A D = x , D B = 3 x and the quarter of the circle has radius C E = 2 x − 2 .
Find A 1 A 2 .
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A F = 2 x − ( 2 x − 2 ) = 2 , G D = 2 3 x , A G = 2 x ⟹
( C D ) 2 = 4 ( x − 1 ) 2 = 1 6 x 2 − 3 2 x + 1 6 = ( 2 x − 2 x ) 2 + 4 3 x 2 = 1 2 x 2 ⟹ x 2 − 8 x + 4 = 0 ⟹ x = 4 ± 2 3
x = 4 − 2 3 ⟹ 2 ( x − 1 ) = 2 ( 3 − 2 3 ) < 0 ∴ choose x = 4 + 2 3
⟹ radius r = 2 x − 2 = 2 ( 3 + 2 3 )
For A 2 :
Let θ = m ∠ D C E
⟹ tan ( θ ) = 3 ⟹ θ = 3 π ⟹ A s e c t o r C D E = 2 1 θ r 2 = 2 1 ( 3 π ) ( 4 ) ( 3 + 2 3 ) 2 =
2 π ( 7 + 4 3 )
and A △ C D I = 2 1 ( 2 3 x ) ( 2 3 x ) = 8 3 3 x 2 = 8 3 3 ( 4 + 2 3 ) 2 = 2 3 3 ( 7 + 4 3 )
Let R 2 be region D E I ⟹ A R 2 = A s e c t o r C D E − A △ C D I = 2 ( 7 + 4 3 ) ( 4 π − 3 3 )
B C = 2 3 x ⟹ B E = 2 3 x − ( 2 x − 2 ) = 2 ( ( 3 − 1 ) x + 1 ) =
2 ( 2 3 + 3 ) and △ A D G ∼ △ H B E ⟹ 3 1 = 2 ( 2 3 + 3 ) H E ⟹ H E = 3 4 3 + 6 = 4 + 2 3 = x and D I = 2 3 x = 3 ( 2 + 3 ) and I E = C E − C I =
( 2 4 − 3 ) x − 2 = 3 + 2 3 ⟹ area of trapezoid
A D H E I = 2 1 ( 5 ) ( 2 + 3 ) ( 3 + 2 3 ) = 2 5 ( 1 2 + 7 3 )
⟹ A 2 = A D H E I − A R 2 = 4 8 + 2 8 3 − ( 1 4 + 8 3 ) π = 2 ( 2 4 + 1 4 3 − ( 7 + 4 3 ) π )
For A 1 :
m ∠ F C D = 6 π ⟹ A s e c t o r C D F = 2 1 ( 6 π ) ( 4 ) ( 3 + 2 3 ) 2 = π ( 7 + 4 3 )
and A △ D G C = 2 3 3 ( 7 + 4 3 )
Let R 1 be region F D G ⟹ A R 1 = A s e c t o r C D F − A △ D G C = 2 ( 7 + 4 3 ) ( 2 π − 3 3 )
and A △ A D G = 8 3 x 2 = 2 3 ( 7 + 4 3 )
⟹ A 1 = A △ A D G − A R 1 = ( 7 + 4 3 ) ( 2 3 − π ) = 2 4 + 1 4 3 − ( 7 + 4 3 ) π
⟹ A 1 A 2 = 2 .
E H ⊥ B C and intersects A B at H :
⟹ ∴ ⟹ ⟹ ∴ C D = r = C E R T △ C D H ≅ R T △ C E H ∠ E C H = ∠ D C H = 2 1 ∠ B C D = 3 0 ° R T △ C D H ≅ R T △ C D A ≅ R T △ C E H A 2 = 2 A 1 A 1 A 2 = 2
Label the two parallel lines to C A be E F and D G and D H the perpendicular line to C A .. By Pythagorean theorem ,
C G 2 + D G 2 ( A D ⋅ cos 3 0 ∘ ) 2 + ( D B ⋅ sin 3 0 ∘ ) 2 4 3 x 2 + 4 9 x 2 x 2 − 8 x + 4 ( x − 4 ) 2 ⟹ x = C D 2 = ( 2 x − 2 ) 2 = 4 x 2 − 8 x + 4 = 0 = 1 2 = 2 3 + 4 Note that C G = D H
Then C E = 2 x − 2 = 4 3 + 6 and B C = A B ⋅ cos 3 0 ∘ = 2 3 x = 8 3 + 1 2 = 2 ⋅ C E . Then the ratio of lengths B E : B G : B C = 2 : 3 : 4 . Let the area of △ A D H , [ADH] = A). then [ B E F ] = 4 A , [ A D G C ] = 1 2 A , A D G C = 7 A , [ D G C H ] = 6 A , and [ C D H ] = 3 A . Let r = 2 x − 2 . Then we have:
⎩ ⎪ ⎨ ⎪ ⎧ A 1 + A 2 = [ A F E C ] − 4 π r 2 = 1 2 a − 4 π r 2 A 2 = [ A D G C ] − [ C D H ] − 1 2 π r 2 = 4 A − 1 2 π r 2 . . . ( 1 ) . . . ( 2 )
( 1 ) − ( 2 ) : A 1 = 8 A − 6 π r 2 ⟹ A 2 A 1 = 2
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Let the point to the left of E be H , and let B E = y .
Since ∠ A B C = 3 0 ° and A B = 4 x , from △ A B C we have A C = 2 x and B C = 2 3 x = 2 x − 2 + y .
By the tangent-secant theorem , ( 3 x ) 2 = y ( 4 x − 4 + y ) .
These two equations solve to x = 4 + 2 3 and y = 6 + 4 3 .
That means A C = 2 x = 2 ( 4 + 2 3 ) = 8 + 4 3 and E C = 2 x − 2 = 2 ( 4 + 2 3 ) − 2 = 6 + 4 3 .
Since ∠ H B E = 3 0 ° and B E = y = 6 + 4 3 , from △ H B E we have H E = 3 3 ( 6 + 4 3 ) = 4 + 2 3 = x = A D .
By the Pythagorean Theorem on △ A D C , D C = A C 2 − A D 2 = ( 8 + 4 3 ) 2 − ( 4 + 2 3 ) 2 = 6 + 4 3 = E C .
By the Pythagorean Theorem on △ H E C , H C = H E 2 + E C 2 = ( 4 + 2 3 ) 2 + ( 6 + 4 3 ) 2 = 8 + 4 3 = A C .
By the Pythagorean Theorem on △ H D C , H D = H C 2 − D C 2 = ( 8 + 4 3 ) 2 − ( 6 + 4 3 ) 2 = 4 + 2 3 = A D = H E .
Since H D = A D = H E , A C = H C , and D C = E C , triangles △ A D C , △ H D C and △ H E C are all congruent to each other by SSS congruency.
By corresponding parts of congruent triangles, ∠ A C D = ∠ H C D = ∠ H C E .
Since A 2 spans exactly twice the triangles and angles of A 1 , A 1 A 2 = 2 .