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Algebra Difficulty 3.2 AMC 10/12 Prove it Canada

A list a1,a2,a3,a4a_1,a_2,a_3,a_4 of rational numbers is
defined so that if one term is equal to rr, then the next term is equal to 1+11+r1 + \dfrac{1}{1+r}. For example, if a3=4129a_3=\dfrac{41}{29}, then $a_4 = 1 + 11\dfrac{1}{1} + (41/29)} =
9970\dfrac{99}{70}.If. If a3=4129$,a_3=\dfrac{41}{29}\$, what is the value of
a1a_1?
A hollow cylindrical tube has a radius of 10 mm and a
height of 100 mm. The tube sits flat on one of its circular faces on a
horizontal table. The tube is filled with water to a depth of hh mm. A solid cylindrical rod has a
radius of 2.5 mm and a height of 150 mm. The rod is inserted into the
tube so that one of its circular faces sits flat on the bottom of the
tube. The height of the water in the tube is now 64 mm. Determine the
value of hh.

Solution

If rr is a term in the
sequence and ss is the next term,
then s=1+11+rs = 1 + \dfrac{1}{1+r}.

This means that $s - 1 =
11+r\dfrac{1}{1+r}andso and so 1s1\dfrac{1}{s-1} = 1+rwhichgives which gives r = 1s1\dfrac{1}{s-1} - 1$.

Therefore, since $a_3 =
4129\dfrac{41}{29},then, then $a_2 =
1a31\dfrac{1}{a_3-1} - 1 = 1(41/29)1\dfrac{1}{(41/29)-1} - 1 = 112/29\dfrac{1}{12/29} - 1 =
2912\dfrac{29}{12} - 1 = 1712\dfrac{17}{12}Further,since$a2=1712$,then Further, since \$a_2 = \dfrac{17}{12}\$, then a_1 = 1a21\dfrac{1}{a_2-1} - 1 = 1(17/12)1\dfrac{1}{(17/12)-1}
- 1 = 15/12\dfrac{1}{5/12} - 1 = 125\dfrac{12}{5} - 1 =
75\dfrac{7}{5}
Initially, the water in the hollow tube forms a cylinder with
radius 10 mm and height hh mm. Thus,
the volume of the water is (10 mm ) 2(h\text{(10 mm ) 2(h} mm}) = 100 h\text{100 h} mm}^3$.

After the rod is inserted, the level of the water rises to 64 mm. Note
that this does not overflow the tube, since the tube’s height is 100
mm.

Up to the height of the water, the tube is a cylinder with radius 10 mm
and height 64 mm.

Thus, the volume of the tube up to the height of the water is (10 mm ) 2(64 mm ) = 6400 mm 3\text{(10 mm ) 2(64 mm ) = 6400 mm 3} This volume consists of the water that is
in the tube (whose volume, which has not changed, is 100 h mm 3\text{100 h mm 3}) and the rod up to
a height of 64 mm.

[[IMAGE0]]

Since the radius of the rod is 2.5 mm, the volume of the rod up to a
height of 64 mm is (2.5 mm ) 2(64\text{(2.5 mm ) 2(64} mm}) = 400\text{400} mm}^3$.

Comparing volumes, 6400\text{6400} mm}^3 =
100 h\text{100 h} mm}^3 + 400\text{400} mm}^3andso and so 100h = 6000whichgives which gives h = 60$.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.