Practice (40)

514

The internal angles of quadrilateral

$ABCD$ form an arithmetic progression. Triangles

$ABD$ and

$DCB$ are similar with

$\angle DBA = \angle DCB$ and

$\angle ADB = \angle CBD$ . Moreover, the angles in each of these two triangles also form an arithemetic progression. In degrees, what is the largest possible sum of the two largest angles of

$ABCD$ ?

517

Let

$ABCDE$ be an equiangular convex pentagon of perimeter

$1$ . The pairwise intersections of the lines that extend the sides of the pentagon determine a five-pointed star polygon. Let

$s$ be the perimeter of this star. What is the difference between the maximum and the minimum possible values of

$s$ .

522

Consider the set of 30 parabolas defined as follows: all parabolas have as focus the point (0,0) and the directrix lines have the form

$y=ax+b$ with a and b integers such that

$a\in \{-2,-1,0,1,2\}$ and

$b\in \{-3,-2,-1,1,2,3\}$ . No three of these parabolas have a common point. How many points in the plane are on two of these parabolas?

525

Let

$ABC$ be a triangle where

$M$ is the midpoint of

$\overline{AC}$ , and

$\overline{CN}$ is the angle bisector of

$\angle{ACB}$ with

$N$ on

$\overline{AB}$ . Let

$X$ be the intersection of the median

$\overline{BM}$ and the bisector

$\overline{CN}$ . In addition

$\triangle BXN$ is equilateral with

$AC=2$ . What is

$BN^2$ ?

532

The shape below can be folded along the dashed lines and taped together along the edges to form a three-dimensional polyhedron. All lengths in the diagram are given in inches. What is the volume of the resulting polyhedron? Express your answer in simplest radical form.

534

Circle $O$ is tangent to two sides of equilateral triangle $XYZ$ . If the two shaded regions have areas 50 $cm^2$ and 100 $cm^2$ as indicated, what is the ratio of the area of triangle $XYZ$ to the area of circle $O$ ? Express your answer as a decimal to the nearest hundredth.

537

Equilateral triangle ABC with side-length 12 cm is inscribed in a circle. What is the area of the largest equilateral triangle that can be drawn with two vertices on segment AB and the third vertex on minor arc AB of the circle? Express your answer in simplest radical form.

541

Four consecutive sides of an equiangular hexagon have lengths of 1, 9, 16 and 4 units, in that order. What is the absolute difference in the lengths of the two remaining sides?

554

A triangle has area

$30$ , one side of length

$10$ , and the median to that side of length

$9$ . Let

$\theta$ be the acute angle formed by that side and the median. What is

$\sin{\theta}$ ?

556

A square region

$ABCD$ is externally tangent to the circle with equation

$x^2+y^2=1$ at the point

$(0,1)$ on the side

$CD$ . Vertices

$A$ and

$B$ are on the circle with equation

$x^2+y^2=4$ . What is the side length of this square?

558

The closed curve in the figure is made up of 9 congruent circular arcs each of length $\frac{2\pi}{3}$ , where each of the centers of the corresponding circles is among the vertices of a regular hexagon of side 2. What is the area enclosed by the curve?

560

Circle

$C_1$ has its center

$O$ lying on circle

$C_2$ . The two circles meet at

$X$ and

$Y$ . Point

$Z$ in the exterior of

$C_1$ lies on circle

$C_2$ and

$XZ=13$ ,

$OZ=11$ , and

$YZ=7$ . What is the radius of circle

$C_1$ ?

562

Triangle

$ABC$ has

$AB=27$ ,

$AC=26$ , and

$BC=25$ . Let

$I$ denote the intersection of the internal angle bisectors of

$\triangle ABC$ . What is

$BI$ ?

566

Distinct planes

$p_1,p_2,....,p_k$ intersect the interior of a cube

$Q$ . Let

$S$ be the union of the faces of

$Q$ and let

$P =\bigcup_{j=1}^{k}p_{j}$ . The intersection of

$P$ and

$S$ consists of the union of all segments joining the midpoints of every pair of edges belonging to the same face of

$Q$ . What is the difference between the maximum and minimum possible values of

$k$ ?

571

A circle of radius 5 is inscribed in a rectangle as shown. The ratio of the length of the rectangle to its width is 2:1. What is the area of the rectangle?

579

What is the area of the polygon whose vertices are the points of intersection of the curves

$x^2 + y^2 =25$ and

$(x-4)^2 + 9y^2 = 81 ?$

582

Two parabolas have equations

$y= x^2 + ax +b$ and

$y= x^2 + cx +d$ , where

$a, b, c,$ and

$d$ are integers, each chosen independently by rolling a fair six-sided die. What is the probability that the parabolas will have a least one point in common?

584

Jesse cuts a circular disk of radius 12, along 2 radii to form 2 sectors, one with a central angle of 120. He makes two circular cones using each sector to form the lateral surface of each cone. What is the ratio of the volume of the smaller cone to the larger cone?

586

Square

$PQRS$ lies in the first quadrant. Points

$(3,0), (5,0), (7,0),$ and

$(13,0)$ lie on lines

$SP, RQ, PQ$ , and

$SR$ , respectively. What is the sum of the coordinates of the center of the square

$PQRS$ ?

588

A trapezoid has side lengths 3, 5, 7, and 11. The sums of all the possible areas of the trapezoid can be written in the form of

$r_1\sqrt{n_1}+r_2\sqrt{n_2}+r_3$ , where

$r_1$ ,

$r_2$ , and

$r_3$ are rational numbers and

$n_1$ and

$n_2$ are positive integers not divisible by the square of any prime. What is the greatest integer less than or equal to

$r_1+r_2+r_3+n_1+n_2$ ?

589

Square

$AXYZ$ is inscribed in equiangular hexagon

$ABCDEF$ with

$X$ on

$\overline{BC}$ ,

$Y$ on

$\overline{DE}$ , and

$Z$ on

$\overline{EF}$ . Suppose that

$AB=40$ , and

$EF=41(\sqrt{3}-1)$ . What is the side-length of the square?

604

Circles

$A, B,$ and

$C$ each have radius 1. Circles

$A$ and

$B$ share one point of tangency. Circle

$C$ has a point of tangency with the midpoint of

$\overline{AB}.$ What is the area inside circle

$C$ but outside circle

$A$ and circle

$B?$

606

Triangle

$ABC$ has side-lengths

$AB = 12, BC = 24,$ and

$AC = 18.$ The line through the incenter of

$\triangle ABC$ parallel to

$\overline{BC}$ intersects

$\overline{AB}$ at

$M$ and

$\overline{AC}$ at

$N.$ What is the perimeter of

$\triangle AMN?$

607

Suppose

$a$ and

$b$ are single-digit positive integers chosen independently and at random. What is the probability that the point

$(a,b)$ lies above the parabola

$y=ax^2-bx$ ?

608

The circular base of a hemisphere of radius

$2$ rests on the base of a square pyramid of height

$6$ . The hemisphere is tangent to the other four faces of the pyramid. What is the edge-length of the base of the pyramid?

PlaneGeometry Triangle AreaMethod SolidGemoetry CoordinatedGeometry GeometryPattern Combinatorics

AMC10/12

With Solutions

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