The area swept out consists of two circular sectors of different radii. The right and left radii are, respectively, (1-l) l /2  and   (1+l) l /2. Noting that one of the signed areas is positive and the other is negative, we get A = ¹/₂ [(1-l) l /2] ² Dq  -  ¹/₂ [(1+l) l /2] ² Dq. Multiplying this out and simplifying yields A = -¹/₂ l l ² Dq.