Mursalin's circle

Geometry Level 3

Let Γ \Gamma be a circle with center O O . Let A B AB be a chord in Γ \Gamma and let T T be a point on A B AB . If A B = 24 , O T = 17 , A T = 7 AB=24, OT=17, AT=7 , and the area of Γ \Gamma is N π N\pi , what is the value of N N ?

This problem is posed by Mursalin H .


The answer is 408.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

8 solutions

Albert Ho
Aug 12, 2013

Use Power of a Point. Extend OT to become a diameter of the circle, with points X and Y on the circle such that X is closer to T and Y is closer to O. Letting "r" be the radius, we have TX = r-17 and TY = r+17, so by PoP, (AT)(TB)=(XT)(TY), which translates to (7)(17) = (r-17)(r+17). The value of N is r^2, which we solve to be 408 .

Idham Muqoddas
Aug 13, 2013

Let C C be a point on A B AB so that O C OC perpendicular to A B AB .

A C = 1 2 A B = 12 AC=\frac12 AB=12

C T = A C A T = 12 7 = 5 CT=AC-AT=12-7=5

O C 2 = O T 2 C T 2 = 1 7 2 5 2 = 264 OC^2=OT^2-CT^2=17^2-5^2=264

Let r r be radii of Γ \Gamma

r 2 = O B 2 = O C 2 + B C 2 = 264 + 144 = 408 r^2=OB^2=OC^2+BC^2=264+144=408

Area of Γ = π r 2 = 408 π \Gamma=\pi r^2=408\pi

Thus, N = 408 N=408

we have the same solution XD

Rindell Mabunga - 7 years, 10 months ago
Ankush Tiwari
Aug 13, 2013

Let the perpendicular from O O on A B AB meet A B AB at M . M.

We know that M M is the mid point of A B . AB.

A M = M B = 12 \Rightarrow AM = MB = 12 and M T = 5 MT =5

In right triangle O M T OMT

O M 2 = O T 2 M T 2 OM^2 = OT^2 - MT^2

O M 2 = 1 7 2 5 2 = 264 \Rightarrow OM^2 = 17^2 - 5^2 = 264

In right triangle O M B OMB

O B 2 = O M 2 + B M 2 = 264 + 1 2 2 = 408 OB^2 = OM^2 + BM^2 = 264 + 12^2 = 408

Area of the circle = π O B 2 = 408 π \pi * OB^2 = 408\pi

N = 408 \Rightarrow N = 408

Nathaniel Lovin
Aug 12, 2013

AB - AT = 24 - 7 = 17 = TB

This shows that BTO is a isosceles triangle (because OT and BT are equal).

Since OA and OB are both radiuses of the triangle, OAB is also isosceles.

Since angle OBA and OBT are the same angle to each other, they are congruent to each other. By the properties of isosceles triangles, OBA is also congruent to OAB and OBT is congruent to TOB. Therefore OAB is congruent to TOB.

Because there are two angles are congruent between triangles BTO and OAB, the third is as well, and the triangles are similar. so the two equal sides of OAB (the radii) are proportional to the 17, and the radius that makes up the third side of BTO is proportional to the third side of OAB (AB, which is 24).

r 24 = 17 r \frac{r}{24} = \frac{17}{r}

r 2 = 17 × 24 r^{2} = 17 \times 24

since the area of a triangle is π r 2 πr^{2} , N = r 2 r^{2} , so N = 408.

And I mean circle, not triangle in the last line. oops.

Nathaniel Lovin - 7 years, 10 months ago
Jonathan Fang
Aug 12, 2013

Extend the segment O T OT to create a diameter that meets the circle at points C C and D D with T T being between C C and O O . Calling C T = x CT=x , you can use the intersecting chords theorem to solve for x x . x ( 34 + x ) = 17 7 x(34+x)=17*7 x 2 + 34 x 119 = 0 x^2+34x-119=0 x = 34 ± 1632 2 x=\frac{-34 \pm \sqrt{1632}}{2} x = 17 + 2 102 x=-17 + 2\sqrt{102} The radius of the circle is x + 17 x+17 or 2 102 2\sqrt{102} and the area will be ( 2 102 ) 2 π (2\sqrt{102})^2\pi or 408 π 408\pi

Victor Britto
Aug 16, 2013

We have two segments, AB and the diameter (we can call FG). The meet each other in T point. So, we know AT=7 and TB = 24 - 7 = 17. We also know FT= r+17 and TG= r-17. Using the fact that the product of the two parts of the chord is equal to the product of the two parts of the other chord, we have: AT . TB = FT . FG => 7 . 17 = (r+17)(r-17) => 119 = r² - 17² ...... *[because (a+b)(a-b) = a²-b²] => 119 = r² -289 => 408 = r² . Once the area of a circle is given by π . r² , the area of Γ can be written as 408π, so 408 =N.

Danny He
Aug 15, 2013

Firstly, N = r 2 N =r^2 so we just need to work out the radius.

Note that triangle A B O ABO is an isosceles, which means that the altitude passing through O O is also the perpendicular bisector of A B AB .

Let the foot of this perpendicular be M M . Clearly M M lies on the same side of T T as B B , so we now know that T M = 17 12 = 5 TM = 17-12 = 5

M T O MTO is a right angle triangle, with sides 5 , 17 , 5, 17, and h h , where h h is the height of triangle A B O ABO . h 2 = 1 7 2 5 2 h^2 = 17^2-5^2

Now going back to A B O ABO , we can see that A O M AOM is also a right angle triangle with sides 12 , h , 12,h, and r r , where r r is the radius of the circle). Now we have r 2 = 1 7 2 5 2 + 1 2 2 = 408 r^2 = 17^2-5^2+12^2 = 408

So N = 408 N=408

Harrison Lian
Aug 14, 2013

Drop the altitude from O O to A B AB and call that C C . We know the line that is the perpendicular bisector of a chord passes through the center. This implies at A C = B C = 12 T C = 5 AC=BC=12 \implies TC=5 . Use the Pythagorean Theorem to find O T OT , which is 264 \sqrt{264} . Use it again to find the radius: 264 + 144 = 408 = r 2 264+144=408=r^2 .

That implies our answer is 408 \boxed{408}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...