Calculating Magnitude of Vector Product of Two Vectors

[dmsgo89]

Homework Statement

For the two vectors in the figure , find the magnitude of the vector product lAl x lBl.
http://session.masteringphysics.com/problemAsset/1040325/3/yf_Figure_1_29.jpg

Homework Equations

lAl*lBl=lAl*lBlsin(alpha)

The Attempt at a Solution

2.8cm * 1.9cm * cos(60) = 2.66cm^2

2.8cm * 1.9cm * cos(120) = -2.66cm^2

Since they made two 60 degree, I thought the alpha was 60 but I got wrong.

Then, I came up with the 120 degree, however it is also wrong.

Where did I do wrong?

Please help me..

 

[ehild]

Why do you use cosine 120 degrees instead of sine?

ehild