Originally Posted by

**waxer1987** I used the cosine law, maybe i'm doing it wrong. This is what I did.

my sketch

A------------------D For some reason I can't draw the pic , but D and C are connected as well as A and B. (parrallelogram)

/ /

/ /7cm

/ /

B-----------------C

9cm

With a line from A to C which is 12.0cm.

And a line from B to D which I need to figure out.

I used Angle BDC

c^2 = b^2 + d^2 - 2bd cosC

144 = 130 - 126cosC

130-144 = -126cosC Should be 144 - 130 = -126 cos C

-14 = -126cosC

-14/-126 = cosC

0.1111... = cosC

c= cos^-1(0.1111...) So cos C = -0.1111...

c= 83 degrees C = 96 degrees

but how can it be 83 degrees if its obtuse? what did i do wrong? did I draw it wrong?