Finding Missing Angles of Parallelogram: What is x and y?  TOPIC_SOLVED

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Finding Missing Angles of Parallelogram: What is x and y?

Postby Hikari_Dreamer_12 on Thu Jul 22, 2010 11:21 pm

Image

Angle A is (2y-5) and Angle D is (x+50). The exterior angle of Angle C is (3x+20). What is x and y?


I am pretty confused with this one. I know that the opposite angles of a parallelogram equal each other, so Angle C is (2y-5). Angle C+the exterior angle of it=180, right? I tried doing this:
(2y-5)+(3x+20)=180
I plugged in (180-x) into y when I wanted to solve for x.
[2(180-x)-5]+(3x+20)=180
360-2x-5+3x+20=180
375+x=180
x=-195
I don't know how I ended up with a negative x value, and I know this won't work for the parallelogram angle. :confused: I think I did something wrong when I replaced the y with 180-x.
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  TOPIC_SOLVED

Postby stapel_eliz on Fri Jul 23, 2010 11:37 am

Hikari_Dreamer_12 wrote:I plugged in (180-x) into y when I wanted to solve for x.

On what basis did you do this? (The substitution would be justified if "x" and "y" were known to be supplementary angles, but I do not see this stipulation in the exercise...?)

What relationship exists between Angle C and the angle exterior to Angle C? What relationship exists between Angle C and Angle A? So what relationship exists between the angle exterior to Angle C and Angle D? Can you use these relationships to create some equations? :wink:
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Re: Finding Missing Angles of Parallelogram: What is x and y

Postby Armando on Fri Sep 17, 2010 10:39 pm

We know that the exterior angle of angle C is equal to the measurement of angle D, due to the opposite interior angle theorem.



Move x coefficients to one side, all other terms to the other



Simplify



We also know that angle D and angle A are complementary (same-side interior angle theorem), so we can use this equation:



Substitute & simplify



Subtract and solve





Let's verify our result









So there you have it. and
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Re: Finding Missing Angles of Parallelogram: What is x and y

Postby aquiles on Sat Dec 04, 2010 6:08 pm

Hi guys, I know this is a very late review, but I cannot figure how y became 55
either way you solve the equation ( the original and the suggested) y is 60 and not 55
checking your original equation:
(2y - 5) + (3x + 20) = 180 original
(2*60 - 5) + (3 *15 + 20) = 120 - 5 + 45 + 20 =115 + 65 = 180 :lol:
love the story-problem :clap:
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