Paralellbomb

Geometry Level 2

A wire of length 50 50 c m cm is to be bent in the form of a parallelogram of area 50 50 c m 2 cm^2 . If the angle between the adjacent sides is 3 0 0 30^{0} , then the dimensions of the parallelogram are :

17 17 c m cm , 8 8 c m cm 15 15 c m cm , 10 10 c m cm 20 20 c m cm , 5 5 c m cm 13 13 c m cm , 12 12 c m cm

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2 solutions

Maggie Miller
Jul 17, 2015

Let x x be one side length of the parallelogram in cm. Since the perimeter of the paralellogram is 50 50 cm, the other side length is 25 x 25-x cm.

Let h h be the height of the paralellogram with respect to the base of length x x . Since the area of the parellogram is 50 50 cm 2 ^2 , h = 50 x h=\frac{50}{x} cm.

We have ( 25 x ) sin ( π 6 ) = 50 x (25-x)\cdot \sin\left(\frac{\pi}{6}\right)=\frac{50}{x} . Multiplying by 2 x 2x on both sides, we find 25 x x 2 = 100 25x-x^2=100 , so ( x 5 ) ( x 20 ) = 0 (x-5)(x-20)=0 . Then x = 5 x=5 or x = 20 x=20 . In either case, the dimensions of the parallelogram are 5 5 cm and 20 20 cm.

3 0 o 30^{o} = π 6 \frac{\pi}{6} @Maggie Miller ??? If yes, how?

Sahba Hasan - 5 years, 10 months ago

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I was just using radians because they're more convenient. 3 0 = π 6 30^{\circ}=\frac{\pi}{6} radians; just unit conversion.

Radians are the standard unit for angles; that's what calculators use.

Maggie Miller - 5 years, 10 months ago

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Any other way to do this without using radians @Maggie Miller ??

Sahba Hasan - 5 years, 10 months ago

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@Sahba Hasan I mean I could have written 3 0 30^{\circ} instead of π 6 \frac{\pi}{6} , radians don't change anything. I just didn't know how to type the degree symbol in my paint program.

Maggie Miller - 5 years, 10 months ago

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@Maggie Miller ohh.. i understood it. Thanks...

Sahba Hasan - 5 years, 10 months ago

The angle between adjacent sides is not just 3 0 30^\circ , it is also 15 0 150^\circ depending on which sides. Maybe it should say "the angle between two pairs of adjacent sides."

Marta Reece - 4 years, 1 month ago
Matthew Accisano
Jul 21, 2020

Let a represent the length of one side and let b represent the length of the other side of the parallelogram. You then have a pair of sides of length a and a pair of sides of length b as shown below.

Since you are given the perimeter of 50 you have 2(a+b) = 50.

You are given that one angle is 30 degrees (angle NMQ below) and can determine angle MNP is 150 degrees because MNP and NMQ are supplementary.

Drop a perpendicular from point M to point R and extend segment PN to point R (shown below). Angle MNR is supplementary to angle MNP so angle MNR = 30. This creates 30-60-90 triangle MNR. Through properties of 30-60-90 MR is 1 2 \frac{1}{2} b.

MR is also the height of the parallelogram which gives the equation 1 2 \frac{1}{2} ba = 50. (Area of 50 was given) .

Solving for a you get a = 100 b \frac{100}{b}

Substituting for a in 2(a+b)=50 gets you 2( 100 b \frac{100}{b} +b)=50

2( 100 b \frac{100}{b} +b)=50

100 b \frac{100}{b} + b = 25

100 + b^2 = 25b

b^2 - 25b + 100 = 0

(b - 5)(b-20) = 0

So b is either 5cm or 20cm making a either 20cm or 5cm respectfully. Either way the dimensions are 5cm and 20 cm.

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