First, we will prove that this convex quadrilateral has exactly 1 angle <90* it isn't hard , we will just see some cases 1)exactly 3 angles are bigger than 90 2)exactly 2 We use these facts 1.if the two angles which has common side are $<=90$ then every inner point is observable from that side 2.if one is greater but the second is less, when we outline height from the vertex of greater angle to common side then all inner points of small quadrilateral are observable from that side other facts are same to these ones .So if you draw picture you will understand everything ))