# Thread: sides of a parellelogram

1. ## 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

2. 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 $\displaystyle a=10,\;b=15$ and $\displaystyle \angle C = 25^o$
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: .$\displaystyle c^2 \;=\;a^2+b^2 - 2ab\cos C$

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

Therefore: .$\displaystyle c \;=\;7.287500524$

3. Originally Posted by myoplex11
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:

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

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

4. thanks i got it i just plugged the angles in radian mode

5. Originally Posted by myoplex11
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:

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

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

$\displaystyle c^2=\;53.10766389$

$\displaystyle c= \sqrt {53.10766389}$

$\displaystyle c= 7.2875$

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

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

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

$\displaystyle c^2=\;596.8923361$

$\displaystyle c=\sqrt {596.8923361}$

$\displaystyle c= 24.4314$

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