Specifically, it is like this.

Square the vector m and get the module length of 9/8.

Accordingly, formulas of trigonometric functions is: {cos [(a-b/2]} 2 * I 2+(5/4) {sin [(a+b)/2]} 2 * J 2.

(The product of vectors I and J in the middle is equal to zero.)

Then, using the simplified formula (angle expansion), we can get:-(5/8) cos (a+b)+(1/2) cos (a-b)+9/8 = 9/8.

Transferable items are: cosAcosB=9sinAsinB.

Just divide the form on the left to the right.