If the mark on the vector is not entered well, you won't lose.

The sizes of |a-b| and |a+b| can be compared, but if we want to discuss the situation separately, it is wrong to write like you.

Because ab is not linear, the included angle of ab is 0.

When the angle between ab is acute, it is 0.

When the included angle between ab is obtuse, it is π/2.

|a-b|=|a+b| If and only if θ=π/2.