In the figure, M is the mid-point of AB and N is the mid-point o

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 Multiple Choice QuestionsLong Answer Type

41. ABCD is a quadrilateral and O is any point in its plane. Show that it OA with rightwards arrow on top space plus space OB with rightwards arrow on top space plus space OC with rightwards arrow on top space plus space OD with rightwards arrow on top space equals space 0 with rightwards arrow on top then O is the point of intersection of lines joining the middle points of the opposite sides of ABCD.
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42. For any two vectors straight a with rightwards arrow on top space and space straight b with rightwards arrow on top, prove that
(i)        open vertical bar straight a with rightwards arrow on top space plus space straight b with rightwards arrow on top close vertical bar space less or equal than space open vertical bar straight a with rightwards arrow on top close vertical bar space plus space open vertical bar straight b with rightwards arrow on top close vertical bar              (ii)     open vertical bar straight a with rightwards arrow on top space minus space straight b with rightwards arrow on top close vertical bar space less or equal than space open vertical bar straight a with rightwards arrow on top close vertical bar space plus space open vertical bar straight b with rightwards arrow on top close vertical bar         (iii) open vertical bar straight a with rightwards arrow on top space minus space straight b with rightwards arrow on top close vertical bar space space space greater-than or slanted equal to space space open vertical bar straight a with rightwards arrow on top close vertical bar space minus space open vertical bar straight b with rightwards arrow on top close vertical bar

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43.

If straight c with rightwards arrow on top space equals space 3 straight a with rightwards arrow on top space plus space 4 straight b with rightwards arrow on top and 2 straight c with rightwards arrow on top space equals space straight a with rightwards arrow on top space minus space 3 straight b with rightwards arrow on top, show that
(i) straight c with rightwards arrow on top space and space straight a with rightwards arrow on top have the same direction and open vertical bar straight c with rightwards arrow on top close vertical bar space greater than space open vertical bar straight a with rightwards arrow on top close vertical bar
(ii) straight c with rightwards arrow on top space and space straight b with rightwards arrow on top have opposite direction and 

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 Multiple Choice QuestionsShort Answer Type

44. Show that the line joining the middle points of the consecutive sides of a quadrilateral is a parallelogram.
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 Multiple Choice QuestionsLong Answer Type

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45. In the figure, M is the mid-point of AB and N is the mid-point of CD and O is the mid-point of MN. Prove that
(i) OA with rightwards arrow on top space plus space OB with rightwards arrow on top space plus space OC with rightwards arrow on top space plus space OD with rightwards arrow on top space equals space straight O with rightwards arrow on top
(ii) BC with rightwards arrow on top space plus space AD with rightwards arrow on top space equals space 2 space MN with rightwards arrow on top


We have
                          OA with rightwards arrow on top space plus space AM with rightwards arrow on top space equals space OM with rightwards arrow on top                             ...(1)
and                   OB with rightwards arrow on top space plus space BM with rightwards arrow on top space equals space OM with bar on top                               ...(2)
Adding (1) and (2) we get,
           OA with rightwards arrow on top space plus space OB with rightwards arrow on top space plus space AM with rightwards arrow on top space plus space BM with rightwards arrow on top space equals space 2 space OM with rightwards arrow on top
OA with rightwards arrow on top space plus OB with rightwards arrow on top space plus space AM with rightwards arrow on top space minus space AM with rightwards arrow on top space equals space 2 space OM with rightwards arrow on top space space space left square bracket because space space AM with rightwards arrow on top space equals space minus BM with rightwards arrow on top right square bracket
therefore space space space space space space space space OA with rightwards arrow on top space plus space OB with rightwards arrow on top space equals space 2 space OM with rightwards arrow on top space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space... left parenthesis 3 right parenthesis
Similarly space space space space OC with rightwards arrow on top space plus space OD with rightwards arrow on top space equals space 2 space ON with rightwards arrow on top space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space... left parenthesis 4 right parenthesis space space space space space space space space space space space space space space space space space space space space space space
Adding (3) and (4), we get,
               OA with rightwards arrow on top plus OB with rightwards arrow on top plus OC with rightwards arrow on top plus OD with rightwards arrow on top space equals space 2 space open parentheses OM with rightwards arrow on top plus ON with rightwards arrow on top close parentheses
                                                  equals space 2 space left parenthesis OM with rightwards arrow on top space minus space OM with rightwards arrow on top right parenthesis            open square brackets because space space space OM with rightwards arrow on top space equals space minus ON with rightwards arrow on top close square brackets
                                                   equals space 2 space left parenthesis 0 with rightwards arrow on top right parenthesis
         therefore space space space space OA with rightwards arrow on top space plus space OB with rightwards arrow on top space plus OC with rightwards arrow on top space plus space OD with rightwards arrow on top space equals space 0 with rightwards arrow on top
Also,     BC with rightwards arrow on top space equals space BM with rightwards arrow on top space plus space MN with rightwards arrow on top space plus space NC with rightwards arrow on top space
and       AD with rightwards arrow on top space equals space AM with rightwards arrow on top space plus space MN with rightwards arrow on top space plus space ND with rightwards arrow on top
Adding,                    BC with rightwards arrow on top space plus space AD with rightwards arrow on top space equals space left parenthesis BM with rightwards arrow on top space plus space AM with rightwards arrow on top right parenthesis space plus space 2 space MN with rightwards arrow on top space plus space left parenthesis NC with rightwards arrow on top space plus space ND with rightwards arrow on top right parenthesis
                                                  equals space open parentheses negative AM with rightwards arrow on top space plus space AM with rightwards arrow on top close parentheses space plus space 2 space MN with rightwards arrow on top space plus left parenthesis space stack NC space with rightwards arrow on top space minus space NC with rightwards arrow on top right parenthesis
              therefore space space space space space space space space BC with rightwards arrow on top space plus space AD with rightwards arrow on top space equals space 0 with rightwards arrow on top space plus space 2 space MN with rightwards arrow on top space plus space 0 with rightwards arrow on top
                                     equals 2 space MN with rightwards arrow on top
            

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46. Prove by vector method that the line segment joining the mid-points of the diagonals of trapezium is parallel to the parallel sides and equal to help of there difference.
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47. ABCD is a parallelogram. If L and M are the mid-point of BC and DC respectively, then express AL with rightwards arrow on top space and space AM with rightwards arrow on top in terms of AB with rightwards arrow on top space and space AD with rightwards arrow on top.
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48. ABCDEF is a regular hexagon. Show that
(i)  AB with rightwards arrow on top space plus space AC with rightwards arrow on top space plus space AD with rightwards arrow on top space plus space AE with rightwards arrow on top space plus space AF with rightwards arrow on top space equals space 3 space AD with rightwards arrow on top
(ii) AB with rightwards arrow on top space plus space AC with rightwards arrow on top space plus space AD with rightwards arrow on top space plus space AE with rightwards arrow on top space plus space AF with rightwards arrow on top space equals space 6 space AO with rightwards arrow on top
where O is centre of the hexagon.
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49. Prove that the sum of all the v ectors from the centre of a regular octagon to its vertices is the zero vector.
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50. Show that the sum of three vector determined by the medians of a triangle directed from the vertices is zero.
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