Given the vectors and , the statement " " is a ______ condition for "the angle between and is acute".
Problem 209
Pick one
Official solution
Let's calculate the dot product of vectors and :
For the vectors and to be collinear with the same direction, we must have , with .
Setting up the equations:
we find that . When , the condition is satisfied, but in this case, the angle between the two vectors is , so the angle being acute is not true.
If and form an acute angle, then their dot product must be positive:
Thus, must be true. This implies that " " is a necessary but not sufficient condition for the angle between and to be acute.
Therefore, the correct answer is:
This problem assesses the understanding of necessary and sufficient conditions along with the relationship between the angle of vectors and their dot product.