In the circle above chords A C and B E intersect at D , where B D = a , D E = a + 4 and D C = a + 2 .
If the radius of the circle r = 2 5 and a can be expressed as a = α β + α α α λ − α , where α , β and λ are coprime positive integers, find α + β + λ .
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a = 6
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Thanks. You should also inform @Rocco Dalto , who set the problem.
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I didn't realize that ( 2 + 6 ) 2 = 4 + 4 6 + 6 = 1 0 + 4 6 ⟹ 2 + 6 = 1 0 + 4 6 .
Although, I stated in the problem that a can be expressed as a = α β + α α α λ − α so that the problem is still correct.
Let ( x 0 , y 0 ) be the center of the circle.
x 0 − ( a + 4 ) ) 3 + y 0 2 = r 2
( x 0 + a ) 2 + y 0 2 = r 2
Subtracting the two equations we obtain:
4 ( a + 2 ) x 0 = 8 ( a + 2 ) ⟹ x 0 = 2 .
For y 0 :
Using x 0 = 2 we have:
( a + 2 ) 2 + y 0 2 = r 2
4 + ( y 0 + ( a + 2 ) ) 2 = r 2
Subtracting the two equations we obtain:
− 2 ( a + 2 ) y 0 = 4 ⟹ y 0 = a + 2 − 2
⟹
( a + 2 ) 2 + ( a + 2 ) 2 4 = 2 0 ⟹ ( a + 2 ) 4 − 2 0 ( a + 2 ) 2 − 4 = 0 ⟹
a = ± 1 0 ± 4 6 − 2
The only positive real root is a = 1 0 + 4 6 − 2 = 2 ∗ 5 + 2 2 2 ∗ 3 − 2 =
α ∗ β + α α α ∗ λ − α ⟹ α + β + λ = 1 0 .
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Let the center of the circle be the origin O ( 0 , 0 ) and the line B E is b above O . Then in coordinates, E ( a + 2 , b ) and C ( − 2 , b − ( a + 2 ) ) . By Pythagorean theorem, we have:
{ O E 2 : O C 2 : ( a + 2 ) 2 + b 2 = r 2 ( − 2 ) 2 + ( b − ( a + 2 ) ) 2 = r 2
⟹ ( a + 2 ) 2 + b 2 ⟹ 2 b ( a + 2 ) b = 4 + ( b − ( a + 2 ) ) 2 = 4 + b 2 − 2 b ( a + 2 ) + ( a + 2 ) 2 = 4 = a + 2 2
Putting in O E 2 :
( a + 2 ) 2 + ( a + 2 ) 2 4 = r 2 ( a + 2 ) 4 − 2 0 ( a + 2 ) 2 + 4 ⟹ ( a + 2 ) 2 a = 0 = 2 2 0 + 2 0 2 − 1 6 = 1 0 + 4 6 = 1 0 + 4 6 − 2 = 2 + 6 − 2 = 6 In response to Peter Csorba’s comments
Therefore α + β + γ = 2 + 5 + 3 = 1 0 .