The number of points, having both co-ordinates as integers, that lie in the interior of the triangle with vertices (0,0),(0,41) and (41,0) is
(A)
820
(B)
780
(C)
901
(D)
861
Answer:B
Two sides of a rhombus are along the lines, x−y+1=0 and 7x−y−5=0. If its diagonals intersect at (1,2), then which one of the following is a vertex of this rhombus?