He said, that one of the angel of that this formula, is the angel between the normal of the slip plane and the axis of the rode, later he said that the same angel is the angel between slip direction and the axis of the rod, but this contradict itself seen we have got task’s were the slip direction isn’t in the slip plane.
The slip direction is normal to the slip plane. So the two relations that you mentioned are equivalent. It is not saying that the slip direction is the same. You said the normal of the slip plane and the slip direction: these are the same.
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