## 2009AMC12B真题

#### 答案解析请参考文末

## Problem 1

Each morning of her five-day workweek, Jane bought either a -cent muffin or a -cent bagel. Her total cost for the week was a whole number of dollars. How many bagels did she buy?

## Problem 2

Paula the painter had just enough paint for identically sized rooms. Unfortunately, on the way to work, three cans of paint fell off her truck, so she had only enough paint for rooms. How many cans of paint did she use for the rooms?

## Problem 3

Twenty percent off is one-third more than what number?

## Problem 4

A rectangular yard contains two flower beds in the shape of congruent isosceles right triangles. The remainder of the yard has a trapezoidal shape, as shown. The parallel sides of the trapezoid have lengths and meters. What fraction of the yard is occupied by the flower beds?

## Problem 5

Kiana has two older twin brothers. The product of their ages is . What is the sum of their three ages?

## Problem 6

By inserting parentheses, it is possible to give the expressionseveral values. How many different values can be obtained?

## Problem 7

In a certain year the price of gasoline rose by during January, fell by during February, rose by during March, and fell by during April. The price of gasoline at the end of April was the same as it had been at the beginning of January. To the nearest integer, what is ?

## Problem 8

When a bucket is two-thirds full of water, the bucket and water weigh kilograms. When the bucket is one-half full of water the total weight is kilograms. In terms of and , what is the total weight in kilograms when the bucket is full of water?

## Problem 9

Triangle has vertices , , and , where is on the line . What is the area of ?

## Problem 10

A particular -hour digital clock displays the hour and minute of a day. Unfortunately, whenever it is supposed to display a , it mistakenly displays a . For example, when it is 1:16 PM the clock incorrectly shows 9:96 PM. What fraction of the day will the clock show the correct time?

## Problem 11

On Monday, Millie puts a quart of seeds, of which are millet, into a bird feeder. On each successive day she adds another quart of the same mix of seeds without removing any seeds that are left. Each day the birds eat only of the millet in the feeder, but they eat all of the other seeds. On which day, just after Millie has placed the seeds, will the birds find that more than half the seeds in the feeder are millet?

## Problem 12

The fifth and eighth terms of a geometric sequence of real numbers are and respectively. What is the first term?

## Problem 13

Triangle has and , and the altitude to has length . What is the sum of the two possible values of ?

## Problem 14

Five unit squares are arranged in the coordinate plane as shown, with the lower left corner at the origin. The slanted line, extending from to , divides the entire region into two regions of equal area. What is ?

## Problem 15

Assume . Below are five equations for . Which equation has the largest solution ?

## Problem 16

Trapezoid has , , , and . The ratio is . What is ?

## Problem 17

Each face of a cube is given a single narrow stripe painted from the center of one edge to the center of its opposite edge. The choice of the edge pairing is made at random and independently for each face. What is the probability that there is a continuous stripe encircling the cube?

## Problem 18

Rachel and Robert run on a circular track. Rachel runs counterclockwise and completes a lap every seconds, and Robert runs clockwise and completes a lap every seconds. Both start from the start line at the same time. At some random time between minutes and minutes after they begin to run, a photographer standing inside the track takes a picture that shows one-fourth of the track, centered on the starting line. What is the probability that both Rachel and Robert are in the picture?

## Problem 19

For each positive integer , let . What is the sum of all values of that are prime numbers?

## Problem 20

A convex polyhedron has vertices , and edges. The polyhedron is cut by planes in such a way that plane cuts only those edges that meet at vertex . In addition, no two planes intersect inside or on . The cuts produce pyramids and a new polyhedron . How many edges does have?

## Problem 21

Ten women sit in seats in a line. All of the get up and then reseat themselves using all seats, each sitting in the seat she was in before or a seat next to the one she occupied before. In how many ways can the women be reseated?

## Problem 22

Parallelogram has area . Vertex is at and all other vertices are in the first quadrant. Vertices and are lattice points on the lines and for some integer , respectively. How many such parallelograms are there?

## Problem 23

A region in the complex plane is defined byA complex number is chosen uniformly at random from . What is the probability that is also in ?

## Problem 24

For how many values of in is ? Note: The functions and denote inverse trigonometric functions.

## Problem 25

The set is defined by the points with integer coordinates, , . How many squares of side at least have their four vertices in ?

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