Because the modulus of a times the modulus of b times the cosine between ab is the value of vector a times vector B.

So our idea is: vector A times vector B, divided by the modulus of vector A times the modulus of vector B.

First, multiply the two vectors of ab, that is, (2e 1+e2) times (3e 1+2e2).

It should be noted that when e 1 is multiplied by e2, the remaining chord values are cos60= 1/2.

Then find the value of a, that is, use 2e 1+e2 square, that is, the modulus of a is equal, that is, multiply two A's. Gets the value of

Similarly, the value of b is obtained.

Finally, the included angle of ab is obtained.

You have to work out the answer yourself! ! !