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Mathematical vector problem
Solution: Square both sides of |a-2b|=|2a+3b| to get A2-4a * b+4b2 = 4a2+12a * b+9b2.

Because the modules of A and B are both 3,4, the original formula can be 9-4a * b+64 = 36+12a * b+144, so vector A * vector b=- 107/ 16. Let the angle between a and b be y, cosy.

This kind of problem is relatively simple. Note that the squared flight is vector a* vector B, not simple a* B ... In addition, you need to know the angle between cos vector A and vector B = vector A * vector B/| A | | B |