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help I need to find lengths

 

$A$, $B$, $C$, and $D$ are points on a circle, and segments $\overline{AC}$ and $\overline{BD}$ intersect at $P$, such that $AP=8$, $PC=2$, and $BD=5$. Find $BP$, given that $BP < DP.$

 

 Sep 6, 2023
 #1
avatar+122 
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To find BP, you can use Ptolemy's Theorem which applies to cyclic quadrilaterals. The theorem states that in such a quadrilateral, AC×BD=AB×CD+BC×AD. First, AC=AP+PC=10 and BD=5. We can express AB and AD in terms of BP and DP since AB=BP+DP and AD=BPDP. Applying Ptolemy's theorem: 10×5=(BP+DP)×2+10×(BPDP) 50=2BP+2DP+10BP10DP 50=12BP8DP 12.5=3BP2DP Since BP<DP, DPBP>0. Let x=DPBP. Then DP=BP+x. Substitute into 3BP=12.5+2DP: 3BP=12.5+2(BP+x) 3BP=12.5+2BP+2x BP=12.5+2x BP2x=12.5 Also, x=DPBP: x=DP(12.5+2x) x=12.52x 3x=12.5 x=12.53 x=256 Finally, BP=12.5+2x BP=12.5253 BP=12.53 BP=256 So BP=256.

 Sep 6, 2023
 #3
avatar+170 
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Ptolemy has nothing to do with this problem. Also, 5 - 25/6 = 5/6 < 25/6 so clearly that cannot be BP. 

plaintainmountain  Sep 6, 2023
edited by plaintainmountain  Sep 6, 2023
 #2
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about 2.9289 units

 Sep 6, 2023
 #4
avatar+122 
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my bad you can solve it :D

 Sep 7, 2023
 #5
avatar+130458 
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Let BP  = x   and DP =  5 - x

 

By the Intersecting Chords Theorem we  have

 

DP * PB  = AP * PC

 

(5 - x) * x =  8 * 2

 

5x - x^2  =  16       rearrange  as

 

x^2 - 5x + 16 =  0       

 

We can see that this has no real solution for  x  because the  discriminant is  negative.....

 

cool cool cool

 Sep 7, 2023

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