ABCD is a parallelogram. If L and M are the mid-point of BC and

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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

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

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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.


Take A as the origin. Let straight b with rightwards arrow on top space comma space straight d with rightwards arrow on top be position vectors of B and D respectively such that
    AB with rightwards arrow on top space equals space straight b with rightwards arrow on top space AD with rightwards arrow on top space equals space straight d with rightwards arrow on top
Now,    AL with rightwards arrow on top space equals space AB with rightwards arrow on top space plus space BL with rightwards arrow on top
                  equals space AB with rightwards arrow on top space plus space 1 half BC with rightwards arrow on top
equals space AB with rightwards arrow on top space plus space 1 half AD with rightwards arrow on top space equals space straight b plus fraction numerator straight d with rightwards arrow on top over denominator 2 end fraction
therefore space space space space space space space position space vector space of space straight L space is space straight b with rightwards arrow on top space plus space fraction numerator straight d with rightwards arrow on top over denominator 2 end fraction
Again AM with rightwards arrow on top space equals space AD with rightwards arrow on top space plus space DM with rightwards arrow on top space equals space AD with rightwards arrow on top space plus space 1 half space DC with rightwards arrow on top space equals space AD with rightwards arrow on top space plus space 1 half AB with rightwards arrow on top
                                 equals space straight d with rightwards arrow on top space plus space 1 half space straight b with rightwards arrow on top


AC with rightwards arrow on top space equals space AB with rightwards arrow on top space plus space BC with rightwards arrow on top space equals space AB with rightwards arrow on top space plus space AD with rightwards arrow on top space equals space straight b with rightwards arrow on top space plus space straight d with rightwards arrow on top
                                 ...(1)

AL with rightwards arrow on top space plus space AM with rightwards arrow on top space equals space open parentheses straight b with rightwards arrow on top space plus space fraction numerator straight d with rightwards arrow on top over denominator 2 end fraction close parentheses space plus space open parentheses straight d with rightwards arrow on top space plus space fraction numerator straight b with rightwards arrow on top over denominator 2 end fraction close parentheses space equals space 3 over 2 straight b with rightwards arrow on top space plus space 3 over 2 straight d with rightwards arrow on top
space space space space space space space space space space space space space space space space space space equals space 3 over 2 left parenthesis straight b with rightwards arrow on top space plus space straight d with rightwards arrow on top right parenthesis space equals space 3 over 2 AC with rightwards arrow on top space space space space space space space space space space space space space space space space space space left square bracket because space of space left parenthesis 1 right parenthesis right square bracket

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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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