Show that a quadrilateral is a parallelogram if an only if diago

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 Multiple Choice QuestionsShort Answer Type

111. Show that the three points A (1, –2, –8) , B (5. 0. –2) and C (11, 3. 7) are collinear and find the ratio in which B divides AC.
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112. If Q is the point of intersection of the medians of the triangle ABC, then prove that QA with rightwards arrow on top space plus space QB with rightwards arrow on top space plus space QC with rightwards arrow on top space equals space 0 with rightwards arrow on top.
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113. ABCD is a parallelogram. E, F are mid-points of BC, CD respectively. AE, AF meet the diaginal BD at Q, P respectively. Show that PQ trisects DB.
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114. If straight a with rightwards arrow on top comma space straight b with rightwards arrow on top comma space stack straight c comma with rightwards arrow on top straight d with rightwards arrow on top are any four vectors in 3 - dimensional space with the same initial point and such that 3 space straight a with rightwards arrow on top space space minus space 2 space straight b with rightwards arrow on top space plus space straight c with rightwards arrow on top space minus space 2 space straight d with rightwards arrow on top space equals space 0 with rightwards arrow on top comma space show that terminals, A, B, C, D of these vectors are coplanar. Find the point at which AC and BD meet. Find the ratio in which P divides AC and BD.
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115. Four points A, B, C, D with position vectors straight a with rightwards arrow on top comma space straight b with rightwards arrow on top comma space straight c with rightwards arrow on top comma space straight d with rightwards arrow on top respectively are such that 3 straight a with rightwards arrow on top space minus space straight b with rightwards arrow on top space plus space space 2 straight c with rightwards arrow on top space minus space 4 space straight d with rightwards arrow on top space equals space 0 with rightwards arrow on top. Show that the four points are coplanar. Also, find the position vector of the point of intersection of lines AC and BD.
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 Multiple Choice QuestionsLong Answer Type

116. The mid-points of two opposite sides of a quadrilateral and the mid-points of the diagonals are the vertices of a parallelogram. Prove using vectors.
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 Multiple Choice QuestionsShort Answer Type

117. A point P divides a line segment AB in the ratio A : 1. Give the values of A for which
P lies in between AB and 
(i) nearer A than B    (ii) nearer B than A
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118. A point P divides a line segment AB in the ratio A : 1. Give the values of A for which P lies outside AB and 
(i) nearer A than B    (ii) nearer B than A.
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 Multiple Choice QuestionsLong Answer Type

119. Show that the lines joining the mid-points of the opposite sides of a quadrilateral bisect each other.
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120. Show that a quadrilateral is a parallelogram if an only if diagonals bisect each other.


(i) Assume that quadrilateral ABCD is a parallelogram.
 Let straight a with rightwards arrow on top comma space straight b with rightwards arrow on top comma space straight c with rightwards arrow on top comma space straight d with rightwards arrow on top be the position vectors of A, B, C and D respectively with reference to O as the origin.
because space space ABCD space is space straight a space parallelogram
therefore space space space AB space equals space DC space and space AB thin space vertical line vertical line thin space DC
therefore space space space AB with rightwards arrow on top space equals space DC with rightwards arrow on top
therefore space space space space straight P. straight V. space of space straight B space minus space straight P. straight V. space of space straight A
space space space space space space space space space space space space space space space space space space space space equals space straight P. straight V. space of space straight C space minus space straight P. straight V. space of space straight D
therefore space space space space space straight b with rightwards arrow on top space minus space straight a with rightwards arrow on top space equals space straight c with rightwards arrow on top space minus straight d with rightwards arrow on top
therefore space space space straight b with rightwards arrow on top plus straight d with rightwards arrow on top space equals space straight a with rightwards arrow on top plus straight c with rightwards arrow on top
therefore space space space space equals space fraction numerator straight b with rightwards arrow on top plus straight d with rightwards arrow on top over denominator 2 end fraction space equals fraction numerator straight a with rightwards arrow on top plus straight c with rightwards arrow on top over denominator 2 end fraction

∴ P.V. of mid-point of diagonal BD = P.V. of mid-point of diagonal AC.
∴ diagonal AC and BD bisect each other.
(ii) Assume that the diagonals AC and BD of the quadrilateral bisect each other.
∴   P.V. of mid-point of diagonal BD = P.V. of mid-point of diagonal AC.
therefore space space fraction numerator straight b with rightwards arrow on top plus straight d with rightwards arrow on top over denominator 2 end fraction space equals space fraction numerator straight a with rightwards arrow on top plus straight c with rightwards arrow on top over denominator 2 end fraction
therefore space space space space straight b with rightwards arrow on top space plus space straight d with rightwards arrow on top space equals space straight a with rightwards arrow on top space plus space straight c with rightwards arrow on top
therefore space space space space straight b with rightwards arrow on top space minus space straight a with rightwards arrow on top space equals space straight c with rightwards arrow on top space minus space straight d with rightwards arrow on top
therefore space space space space OB with rightwards arrow on top space minus space OA with rightwards arrow on top space equals space OC with rightwards arrow on top space minus space OD with rightwards arrow on top
therefore space space space space space AB with rightwards arrow on top space equals space DC with rightwards arrow on top
therefore space space space AB space equals space DC space and space AB thin space vertical line vertical line thin space DC
therefore space space space ABCD space is space straight a space parallelogram

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