Constructing this is harder than solving it!

Geometry Level 2

In square ABCD, AD is 4 centimeters, and M is the midpoint of CD. Let O be the intersection of BM and diagonal AC. What is the ratio of OC to OA? If your answer is a b \frac{a}{b} , then find a + b a+b .


The answer is 3.

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5 solutions

Vaibhav Agarwal
Mar 2, 2014

triangle OMC is similar to triangle OAB. and MC:AB=1:2. Therefore, OC:OA=1:2.

good

muhammad azam - 7 years, 3 months ago

great!

Renato Javier - 7 years, 3 months ago
Marta Reece
Mar 24, 2017

Draw a line D N DN to midpoint of A B AB .

The lines M B MB and D N DN divide the diagonal into three equal segments because C M = M D CM=MD implies C O = O P CO=OP and B N = N A BN=NA implies O P = P A OP=PA .

So O C O A = 1 2 \frac{OC}{OA}=\frac{1}{2} .

Crank Tanvir
Jan 10, 2015

By using pythagoras theorem i find the hypotenuse AC=4√2 now, AC= 3 × O C 3\times OC . so, OC= 4 2 3 \frac{4√2}{3} and OA= 2 × O C 2\times OC . so, OA= 8 2 3 \frac{8√2}{3} According to the question OC:OA= 4 2 3 \frac{4√2}{3} : 8 2 3 \frac{8√2}{3} =1:2 so, the answer is 3. Sorry I can't draw the picture.

I did it by making lines of the form y = m x + b y = mx + b for A C \overline{AC} and B M \overline{BM} , and using D, or the bottom left corner of the square, as the origin on a coordinate plane. Then I found the intersection O O of the two lines to be ( 8 3 , 4 3 ) (\frac{8}{3},\frac{4}{3}) . Then I simply found the distance from O O to the x-intercept of A C \overline{AC} , the length of line O C \overline{OC} , which was 4 2 3 \frac{4\sqrt{2}}{3} . Then I found the distance from O O to the y-intercept of A C \overline{AC} , the length of O A \overline{OA} , which was 8 2 3 \frac{8\sqrt{2}}{3} . Dividing them I got 1 2 \frac{1}{2} , so the answer was 1 + 2 = 3 1 + 2 = \boxed{3} .

Eh, coordinate geometry is pretty messy. You can just logic your way through it. But still, great job!

Finn Hulse - 7 years, 3 months ago

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By using pythagoras theorem i find the hypotenuse AC=4√2 now, AC= 3 × O C 3\times OC . so, OC= 4 2 3 \frac{4√2}{3} and OA= 2 × O C 2\times OC . so, OA= 8 2 3 \frac{8√2}{3} According to the question OC:OA= 4 2 3 \frac{4√2}{3} : 8 2 3 \frac{8√2}{3} =1:2 so, the answer is 3. Sorry I can't draw the picture.

Crank Tanvir - 6 years, 5 months ago

it's pretty complex

Mayank Gupta - 7 years, 3 months ago

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By using pythagoras theorem i find the hypotenuse AC=4√2 now, AC= 3 × O C 3\times OC . so, OC= 4 2 3 \frac{4√2}{3} and OA= 2 × O C 2\times OC . so, OA= 8 2 3 \frac{8√2}{3} According to the question OC:OA= 4 2 3 \frac{4√2}{3} : 8 2 3 \frac{8√2}{3} =1:2 so, the answer is 3. Sorry I can't draw the picture.

Crank Tanvir - 6 years, 5 months ago

This is what I exactly do before i know about similarity. But great job!

Ahmad Naufal Hakim - 7 years, 2 months ago

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By using pythagoras theorem i find the hypotenuse AC=4√2 now, AC= 3 × O C 3\times OC . so, OC= 4 2 3 \frac{4√2}{3} and OA= 2 × O C 2\times OC . so, OA= 8 2 3 \frac{8√2}{3} According to the question OC:OA= 4 2 3 \frac{4√2}{3} : 8 2 3 \frac{8√2}{3} =1:2 so, the answer is 3. Sorry I can't draw the picture.

Crank Tanvir - 6 years, 5 months ago

By using pythagoras theorem i find the hypotenuse AC=4√2 now, AC= 3 × O C 3\times OC . so, OC= 4 2 3 \frac{4√2}{3} and OA= 2 × O C 2\times OC . so, OA= 8 2 3 \frac{8√2}{3} According to the question OC:OA= 4 2 3 \frac{4√2}{3} : 8 2 3 \frac{8√2}{3} =1:2 so, the answer is 3. Sorry I can't draw the picture.

Crank Tanvir - 6 years, 5 months ago
Dean Clidoro
Mar 13, 2014

Solution:

  1. find the equations of the line

    a. line AC is an identity with negative y so, x = -y

    using (y-y1) = (y2-y1)(x-x1)/(x2-x1)
    

    b. line BM is y = 2x - 8

  2. Find the intersection

    a. y= -x ; -x = 2x - 8; so x = 8/3 and y = -8/3

3.AC=sqrt((2(4^2 )))=4√2 ; AO= sqrt((2(8/3)^2 ))=8/3 √2

  1. AC - AO = OC = 4√2- 8/3 √2 = 4/3 √2

  2. a = AC ; b =AO ; a/b=1/2 ; a+b=3

By using pythagoras theorem i find the hypotenuse AC=4√2 now, AC= 3 × O C 3\times OC . so, OC= 4 2 3 \frac{4√2}{3} and OA= 2 × O C 2\times OC . so, OA= 8 2 3 \frac{8√2}{3} According to the question OC:OA= 4 2 3 \frac{4√2}{3} : 8 2 3 \frac{8√2}{3} =1:2 so, the answer is 3. Sorry I can't draw the picture.

Crank Tanvir - 6 years, 5 months ago

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