Midpoints in a Square

Geometry Level 3

A B C D ABCD is a square with side length 8 8 . Let M M and N N be the midpoints of sides B C BC and C D CD , respectively, and let θ = M A N \theta=\angle MAN . Then what is the value of sin θ + cos θ ? \sin \theta+\cos \theta?

9 5 \frac{9}{5} 7 5 \frac{7}{5} 6 5 \frac{6}{5} 8 5 \frac{8}{5}

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3 solutions

Noman Burki
Feb 28, 2014

the angle at MAN is 45 take sin 45 and cos 45 u will get 7/5

Raj Sukhdeo
Feb 25, 2014

I wish I can take a pic of my solution and upload here - editor doesn't work for me. Step 1 - just draw a square and state the lengths ,place the midpoints and use simple surds to find the sides of the triangles formed. Step 2 - for the triangle within the square - MN = 4 ROOT 2 and AM is root 80 .The perpendicular bi sector of the triangle (height) is 6 root 2 . step 3 - use the sine rule a/sinA =b/sin B and find for sin x = 3/5 then just draw a right angle using sin x =3/5 and find cos x = 4/5 add and you will get 7/5 Guys can we upload pics as solutions ?

To upload pics, you can load them onto Imgur / Flick, and then use markdown to pull up the picture. For example, the code

   ! [image] (https://ds055uzetaobb.cloudfront.net/avatars-2/resized/80/e6e66b8981c1030d5650da159e79539a.3fe78221256b7adcfbcc8b609332e38a.jpg)

with spaces removed will produce

image image

Give it a try!

Calvin Lin Staff - 7 years, 3 months ago
Tom Engelsman
Nov 15, 2020

Right angle D A B DAB is expressible as π 2 = θ + 2 arctan ( 4 / 8 ) θ = π 2 2 arctan ( 1 / 2 ) . \frac{\pi}{2} = \theta + 2\arctan(4/8) \Rightarrow \theta = \frac{\pi}{2} - 2\arctan(1/2). Thus:

sin θ + cos θ = sin ( π 2 2 arctan ( 1 / 2 ) ) + cos ( π 2 2 arctan ( 1 / 2 ) ) ; \sin\theta + \cos\theta = \sin(\frac{\pi}{2} - 2\arctan(1/2)) + \cos(\frac{\pi}{2} - 2\arctan(1/2));

or cos ( 2 arctan ( 1 / 2 ) ) + sin ( 2 arctan ( 1 / 2 ) ) ; \cos(2\arctan(1/2)) + \sin(2\arctan(1/2));

or 2 cos 2 ( arccos ( 2 / 5 ) 1 + 2 sin ( arcsin ( 1 / 5 ) cos ( arccos ( 2 / 5 ) ; 2\cos^{2}(\arccos(2/\sqrt{5}) - 1 + 2\sin(\arcsin(1/\sqrt{5})\cos(\arccos(2/\sqrt{5});

or 2 ( 4 5 ) 1 + 2 ( 1 5 ) ( 2 5 ) ; 2(\frac{4}{5}) - 1 + 2(\frac{1}{\sqrt{5}})(\frac{2}{\sqrt{5}});

or 12 5 1 ; \frac{12}{5} - 1;

or 7 5 . \boxed{\frac{7}{5}}.

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