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Given a quadrilateral ABCD, a circle is inscribed in the [#permalink]
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11 May 2004, 03:54
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Given a quadrilateral ABCD, a circle is inscribed in the quadrilateral in such a way that it touches all of the four sides. What is the perimeter of the quadrilateral?
(1) AB+BC=10
(2) AB+CD=12



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Re: DS101 [#permalink]
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11 May 2004, 04:08
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hallelujah1234 wrote: Given a quadrilateral ABCD, a circle is inscribed in the quadrilateral in such a way that it touches all of the four sides. What is the perimeter of the quadrilateral?
(1) AB+BC=10 (2) AB+CD=12
2 alone is sufficient, while 1 alone is not.
The reason behind is that for a every quadrilateral to have a circle inscribed inside it is equivalent to AB + CD = AD + BC.
Why this is so? If you mark points, where circle inscribed touches quadr. by A1, B1, C1, D1, then AA1 = AD1, BA1 = BB1, CB1 = CC1, DC1 = DD1 => since AB = AA1 + BA1, BC = BB1 + CB1, CD = CC1 + DC1, DA = DD1 + AD1, AB + CD = BC + AD.
So, opposite sides of quadr. sum to equal values. => P = 2*(AB + CD) = 24.



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In my opinion answer should be (A),
Tangents to circle are perpendicular to the radius, using this property, we can prove that it will be a rectangle in which circle is inscribed.
Hence answer should 2*(AB+BC) = 2*10 = 20
Any comments?



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mba wrote: In my opinion answer should be (A),
Tangents to circle are perpendicular to the radius, using this property, we can prove that it will be a rectangle in which circle is inscribed. Hence answer should 2*(AB+BC) = 2*10 = 20
Any comments?
No, a quadrilateral (such that a circle can be inscribed inside it) is not always rectangle. For instance, consider romb (it has all sides equal, but its angles need not be equal to 90).
.../\.....
../..\....
..\../....
...\/.....



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Answer is A
at least two sides of the quadrilateral will be equal.
A is enough.
B is not enough
Hence the answer is A
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Emmanuel: I still didn't get your logic
Why this is so? If you mark points, where circle inscribed touches quadr. by A1, B1, C1, D1, then AA1 = AD1, BA1 = BB1, CB1 = CC1, DC1 = DD1 => since AB = AA1 + BA1, BC = BB1 + CB1, CD = CC1 + DC1, DA = DD1 + AD1, AB + CD = BC + AD.
How come from above logic you can say (AB+CD=BC+AD)?
(Before joining this group I was under illusion that I am an expert in Maths!!! )



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mba!!!... [#permalink]
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12 May 2004, 01:35
mba wrote: Emmanuel: I still didn't get your logic Why this is so? If you mark points, where circle inscribed touches quadr. by A1, B1, C1, D1, then AA1 = AD1, BA1 = BB1, CB1 = CC1, DC1 = DD1 => since AB = AA1 + BA1, BC = BB1 + CB1, CD = CC1 + DC1, DA = DD1 + AD1, AB + CD = BC + AD. How come from above logic you can say (AB+CD=BC+AD)? (Before joining this group I was under illusion that I am an expert in Maths!!! )
OK, see this:
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Intern
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nice job emmauel..thanks for making it more clear...



Senior Manager
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Thanks, emmanuel.
(Now I wonder how come I didn't see this thing earlier, )



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Wow! I really liked Emmanuel's solution.










