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Math Challenge IB Geometry

 
 
LiHan的头像
Math Challenge IB Geometry
LiHan - 2023年08月16日 Wednesday 18:42
 

How should I solve 9.20, 9.24, and 9.27? My answer for 9.20 is not close to the answer at all, while I don't have a strategy for 9.24 and 9.27.

 
WangDr. Kevin的头像
Re: Math Challenge IB Geometry
WangDr. Kevin - 2023年08月16日 Wednesday 23:25
 

For 9.20, it is easier to help you if you show your solution, especially the answer as a fraction.

9.24: We are looking for the perimeter of this shape which looks like a regular hexagon, except that the corners are rounded.  What we need to find are the points where the rubber band starts to touch the circles, and calculate the straight parts and rounded parts separately.

9.27: If the ratio between the perimeters of two circles is $3:2$, what is the ratio between their radii?  Then, what is the ratio between their areas?  Once we know the ratio between the areas, and given the difference between the areas, you may set up an equation to solve for the areas.

LiHan的头像
Re: Math Challenge IB Geometry
LiHan - 2023年08月17日 Thursday 20:21
 

Thank you for replying. 

For 9.20 the answer I got was (2-(π/4))/2, with R being the variable for the radius of the circle. Through the equation √2(side of triangle)=2R, I found that the side of the triangle is √2R. I simplified the equation (√2R*√2R-(1/4)R^2π)/(√2R*√2R) into the final equation, which does not match up with the answer, 17.

I am confused on 9.24 as how to find the point where the straight line stops and the curve begins.

 Also, I meant to say 9.28, not 9.27. Sorry!

WangDr. Kevin的头像
Re: Math Challenge IB Geometry
WangDr. Kevin - 2023年08月17日 Thursday 22:57
 

9.20: First, the area of the triangle is $\dfrac{\sqrt{2}R\cdot\sqrt{2}R}{2}$.  Then you should have the right answer.  Note that the question says using the approximation $\dfrac{22}{7}$ to represent $\pi$, and write the ratio in the form of $\dfrac{P}{Q}$, and then the final answer is $P+Q$.

9.24: When a straight line is tangent to a circle, it is always useful to connect the center and the tangent point.  This is the radius that is perpendicular to the tangent line.  In this problem, look at one of the six circles that touch the rubber band, and connect its center with the two tangent points.  What angle do these radii form?  Also, look at the straight line segment that is tangent to two of the circles.  After connecting the two radii that are perpendicular to the segment, and connecting the two circle's centers, you are getting a rectangle.  What is the length of this straight segment?

9.28: This problem is not really a hard one.  You need to trace the original point A, and each time the rectangle rolls 90 degrees, the point will either go through a quarter circle path (if it's not the center of rotation), or simply stay put (in the case that it is the center of rotation).  So just carefully trace the path, and calculate the quarter circles, and add them up.