question: public Math_Vector rotate ( double angle ) { double rad = angle * TO_RADIANS ; double cos = Math . cos ( rad ) ; double sin = Math . sin ( rad ) ; double newX = this . x * cos - this . y * sin ; double newY = this . x * sin + this . y * cos ; this . x = newX ; this . y = newY ; return this ; }
target:  rotate the vector using angle