Acute angle, in radians  between the lines whose slope is 2 and the line which is perpendicular to the line 2x−y+5=0  is

The slope of the line  2x−y+5=0  is m=−ab=2  , it is same as the slope of the first line. Hence the angle between the line whose slope is 2 and the line which is perpendicular to the line 2x−y+5=0   is   π2=1.57