Prove that the vertices  and  can form the sides of a trian

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 Multiple Choice QuestionsLong Answer Type

121. Prove, using vectors, that the line segment joining the mid-points of the non-parallel sides of a trapezium is parallel to the bases and is equal to half the sum of their lenghts.
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122. 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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123. If D, E and F are the mid-points of the sides of a triangle ABC, show that 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 equals space OD with rightwards arrow on top space plus space OE with rightwards arrow on top space plus space OF with rightwards arrow on top, where O is any arbitrary point.
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124. The points D, E, F divide the sides BC, CA. AB of a triangle in the ratio 1 : 4, 3 : 2 and 3 : 7 respectively. Show that the sum of the vectors AB with rightwards arrow on top comma space BE with rightwards arrow on top space and space CF with rightwards arrow on top is parallel to CK with rightwards arrow on top comma space where K divides AB in the ratio 1 : 3.
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125. Show that the line joining one vertex of a parallelogram to the mid-point of an opposite side trisects the diagonal and is trisected there at.
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126. If P and Q are the mid-points of the sides AB and CD of a parallelogram ABCD, prove that DP and BQ cut the diagonal AC in its points of trisection which are also the points of trisection of DP and BQ respectively.
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127. Points E and F are taken on the sides BC and CD of a parallelogram ABCD such that open vertical bar BF with rightwards arrow on top close vertical bar thin space colon thin space open vertical bar FC with rightwards arrow on top close vertical bar space equals space straight mu comma space space space space open vertical bar DE with rightwards arrow on top close vertical bar space colon thin space open vertical bar EC with rightwards arrow on top close vertical bar space equals space straight lambdaThe straight lines FD and AE intersect at the point O. Find the ratio open vertical bar FO with rightwards arrow on top close vertical bar space colon thin space open vertical bar OD with rightwards arrow on top close vertical bar.
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128. Prove that the vertices straight a with rightwards arrow on top space equals space 3 space straight i with hat on top space plus space straight j with hat on top space minus space 2 space straight k with hat on top comma space space space straight b with rightwards arrow on top space equals space minus space straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top and straight c with rightwards arrow on top space equals space 4 space straight i with hat on top space minus space 2 space straight j with hat on top space minus space 6 space straight k with hat on top can form the sides of a triangle. Find the lengths of the medians of the triangle.


Let A, B, C be the points whose position vectors are
                                straight a with rightwards arrow on top space equals space 3 space straight i with hat on top space plus space straight j with hat on top space minus space 2 space straight k with hat on top comma space straight b with rightwards arrow on top space equals space minus straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top comma space space space straight c with rightwards arrow on top space equals space 4 space straight i with hat on top space minus space 2 space straight j with hat on top space minus space 6 space straight k with hat on top
   AB with rightwards arrow on top space equals space straight P. straight V. space of space straight B space minus space straight P. straight V. space of space straight A space equals space straight b with rightwards arrow on top minus straight a with rightwards arrow on top space equals space minus 4 space straight i with hat on top space plus 2 space straight j with hat on top space minus space 6 space straight k with hat on top
BC with rightwards arrow on top space equals space straight P. straight V. space of space straight C space minus space straight P. straight V. space of space straight B space equals space straight c with rightwards arrow on top minus straight b with rightwards arrow on top space equals space 5 space straight i with hat on top space minus 5 space straight j with hat on top space minus space 10 space straight k with hat on top
CA with rightwards arrow on top space equals space straight P. straight V. space of space straight A space minus space straight P. straight V. space of space straight C space equals space straight a with rightwards arrow on top space minus space straight c with rightwards arrow on top space equals space minus straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top
therefore space space space AB with rightwards arrow on top space plus space BC with rightwards arrow on top space plus space CA with rightwards arrow on top space equals space 0 with rightwards arrow on top
therefore space space space space ABC space is space straight a space triangle.
space Let space straight D comma space straight E comma space straight F space be space mid minus points space of space sides space BC comma space CA comma space AB space respectively.


∴   position vectors of D, E, F are fraction numerator straight b with rightwards arrow on top plus straight c with rightwards arrow on top over denominator 2 end fraction comma space fraction numerator straight c with rightwards arrow on top plus straight a with rightwards arrow on top over denominator 2 end fraction comma space fraction numerator straight a with rightwards arrow on top plus straight b with rightwards arrow on top over denominator 2 end fraction respectively.

           AD with rightwards arrow on top space equals space straight P. straight V. space of space straight B space minus space straight P. straight V. space of space straight A space equals space fraction numerator straight b with rightwards arrow on top plus straight c with rightwards arrow on top over denominator 2 end fraction minus straight a with rightwards arrow on top
                 equals space 1 half left parenthesis straight b with rightwards arrow on top plus straight c with rightwards arrow on top minus 2 space straight a with rightwards arrow on top right parenthesis
equals space 1 half left parenthesis negative stack straight i space with hat on top plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top space plus space 4 space straight i with hat on top space minus space 2 space straight j with hat on top space minus space 6 space straight k with hat on top space minus space space 6 space straight i with hat on top space minus space 2 space straight j with hat on top space plus space 4 space straight k with hat on top right parenthesis
equals space 1 half left parenthesis negative 3 space straight i with hat on top space minus space straight j with hat on top space plus space 2 space straight k with hat on top right parenthesis
 BE with rightwards arrow on top space equals space straight P. straight V. space of space straight E space minus space straight P. straight V. space of space straight B space equals space fraction numerator straight c with rightwards arrow on top plus straight a with rightwards arrow on top over denominator 2 end fraction minus straight b with rightwards arrow on top
space space space space space space space equals space 1 half left parenthesis straight c with rightwards arrow on top plus straight a with rightwards arrow on top space minus space 2 space straight b with rightwards arrow on top right parenthesis
space space space space space space equals space 1 half left parenthesis 4 space straight i with hat on top space minus space 2 space straight j with hat on top space minus space 6 space straight k with hat on top space plus space 3 space straight i with hat on top space plus space straight j with hat on top space minus space 2 space straight k with hat on top space plus space 2 space straight j with hat on top space minus space 6 space straight j with hat on top space minus space 8 space straight k with hat on top right parenthesis
space space space space space space equals space 1 half left parenthesis 9 space straight i with hat on top space minus space space 7 space straight j with hat on top space minus space 16 space straight k with hat on top right parenthesis

CF with rightwards arrow on top space equals space straight P. straight V. space of space straight F space minus space straight P. straight V. space of space straight C space equals space fraction numerator straight a with rightwards arrow on top plus straight b with rightwards arrow on top over denominator 2 end fraction space minus straight c with rightwards arrow on top
space space space space space space equals space 1 half left parenthesis straight a with rightwards arrow on top plus straight b with rightwards arrow on top minus 2 straight c with rightwards arrow on top right parenthesis
space space space space space equals space 1 half left parenthesis 3 space straight i with hat on top space plus space straight j with hat on top space minus space 2 space straight k with hat on top space minus space straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top space minus space 8 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 12 space straight k with hat on top right parenthesis
space space space space equals space 1 half left parenthesis negative 6 space straight i with hat on top space plus space 8 space straight j with hat on top space plus space 14 space straight k with hat on top right parenthesis
space space space space equals space minus space 3 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 7 space straight k with hat on top

therefore space space space AD space equals space 1 half square root of 9 plus 1 plus 4 end root space equals 1 half square root of 14
space space space space space space space space BE space equals space 1 half square root of 81 plus 49 plus 256 end root space equals 1 half square root of 386
space space space space space space space space CF space equals space square root of 9 plus 16 plus 49 end root space equals square root of 74

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 Multiple Choice QuestionsShort Answer Type

129. Evaluate the scalar product:
left parenthesis 3 straight a with rightwards arrow on top space minus space 5 space straight b with rightwards arrow on top right parenthesis. space space left parenthesis 2 straight a with rightwards arrow on top plus space 7 straight b with rightwards arrow on top right parenthesis
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130.

Find open parentheses straight b with rightwards arrow on top space minus space straight a with rightwards arrow on top close parentheses. space space open parentheses 3 space straight a with rightwards arrow on top space plus space straight b with rightwards arrow on top close parentheses when
    straight a with rightwards arrow on top space equals space straight i with hat on top space minus space 2 space straight j with hat on top space plus space 5 space straight k with hat on top comma space space straight b with rightwards arrow on top space equals space 2 space straight i with hat on top space plus space straight j with hat on top space minus space space 3 space straight k with hat on top

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