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Math Help - sides of a parellelogram

  1. #1
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    sides of a parellelogram

    the diagonals of a parellelogram are 20 cm and 30cm long and intersect at an angle of 25 degrees. Find the sides of the parellelogram?


    i used the law of cosines to find one of the sides but i am getting the wrong answer.i let sides a=10 and b=15 and angle C =25 and used the law of cosines got wrong anwser. Where am i going wrong
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  2. #2
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    Hello, myoplex11!

    The diagonals of a parellelogram are 20 cm and 30cm long
    and intersect at an angle of 25.
    Find the sides of the parellelogram.


    i used the Law of Cosines to find one of the sides but i am getting the wrong answer.
    I let sides a=10,\;b=15 and \angle C = 25^o
    and got the wrong answer. Where am i going wrong?

    It's really hard to see your work from here! .Could you write larger?

    Law of Cosines: . c^2 \;=\;a^2+b^2 - 2ab\cos C

    We have: . c^2 \;=\;10^2 + 15^2 - 2(10)(15)\cos25^o \;=\;53.10766389

    Therefore: . c \;=\;7.287500524

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  3. #3
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    Quote Originally Posted by myoplex11 View Post
    the diagonals of a parellelogram are 20 cm and 30cm long and intersect at an angle of 25 degrees. Find the sides of the parellelogram?


    i used the law of cosines to find one of the sides but i am getting the wrong answer.i let sides a=10 and b=15 and angle C =25 and used the law of cosines got wrong anwser. Where am i going wrong
    Edit: Soroban, you're too fast for me.

    What answer did you get for c? Did you use this:

    c^2=a^2+b^2-2ab \cos C

    c^2=10^2+15^2-2(10)(15) \cos 25
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  4. #4
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    thanks i got it i just plugged the angles in radian mode
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  5. #5
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    Quote Originally Posted by myoplex11 View Post
    the diagonals of a parellelogram are 20 cm and 30cm long and intersect at an angle of 25 degrees. Find the sides of the parellelogram?


    i used the law of cosines to find one of the sides but i am getting the wrong answer.i let sides a=10 and b=15 and angle C =25 and used the law of cosines got wrong anwser. Where am i going wrong
    Use cosine law:

    c^2=a^2+b^2-2ab \cos C

    c^2=10^2+15^2-2(10)(15) \cos 25

    c^2=\;53.10766389

    c= \sqrt {53.10766389}

    c= 7.2875

    For calculating other side of parallelogram,
    a = 10, b = 15 and angle = 180 - 25 = 155

    c^2=a^2+b^2-2ab \cos C

    c^2=10^2+15^2-2(10)(15) \cos 155

    c^2=\;596.8923361

    c=\sqrt {596.8923361}

    c= 24.4314

    So, sides of parallelogram are 7.2875 cm and 24.4314 cm.
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