1. The problem statement, all variables and given/known data I did the rest of a problem right.. It asks to determine whether the parallelogram is a rectangle, meaning right angles.. A(2,-1,4), B(3,1,2), C(0,5,6), D(-1,3,8) 2. Relevant equations cos theta = dot product of u and v divided by magnitude of u and v 3. The attempt at a solution I drew a sketch parallelogram, with A at Northwest, B at NE, C at SE, and D at SW.. blah blah I got vector AD and BC to be <-3,4,4> and vectors AB and DC to be <1,2,-2> mag of AB/BC is square root 41, etc. if you can help me, you should know how I got there. now.. cos theta = [(-3*1) +(4*2) + (4*-2)]/[square root of 41*square root of 9] [-3]/[square root 369]=cos theta theta=?98.9??? The back of my book says 81.02 degrees..