I've attempted this problem 8 times and my answers are
still wrong. I used the equation for the angle between planes. Am I doing
something wrong?
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Well, 142 radians is a lot of radians and 2.49 degrees is very few degrees. That should make you suspicious about which one is which. In fact, 2.49 radians divided by π x 180 degrees = 142 degrees, whereas 142 radians divided by π x 180 degrees amounts to 8136 degrees. So: at the very least you have degrees and radians reversed.
Now let's see, you didn't show me how you computed your angles but you should have had:
cos(θ)=u.v/(|u| |v|)
=<4,-4,-1>.<4,-4,-3>/(√(4^2+(-4)^2+(-1)^2)√(4^2+(-4)^2+(-3)^2))
=(16+16+3)/(√(16+16+1)√(16+16+9))
=39/(√37√45)
=0.956
which is a number very close to 1, hence we expect that θ is close to zero. In fact computing the arccos what we arrive at is θ=0.30 radians, which is about 17.1 degrees.
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