Maths-
General
Easy

Question

The points with position vectors bar a plus bar b comma space bar a minus bar b and a with not stretchy bar on top plus lambda b with not stretchy bar on top are collinear for

  1. Only integrals values of λ
  2. no value of λ
  3. all real values of λ
  4. Only rational values of λ

hintHint:

We are given position vectors of three points. We have to find the value of λ such that the vectors are parallel to each other. Vectors are parallel to each other when they are scalar multiple of each other.

The correct answer is: all real values of λ


    Let the position vectors of the three vectors be denoted as follows:
    S with rightwards arrow on top space equals a with rightwards arrow on top space plus b with rightwards arrow on top
T with rightwards arrow on top space equals a with rightwards arrow on top minus b with rightwards arrow on top
U with rightwards arrow on top space equals a with rightwards arrow on top plus lambda b with rightwards arrow on top
    Now the vector from point S to point T will be
    table attributes columnalign left end attributes row cell stack S T with rightwards arrow on top equals T with rightwards arrow on top minus S with rightwards arrow on top end cell row cell equals a with rightwards arrow on top minus b with rightwards arrow on top minus open parentheses a with rightwards arrow on top plus b with rightwards arrow on top close parentheses end cell row cell equals negative 2 b with rightwards arrow on top end cell end table
    The vector from point T to point U will be
    stack T U with rightwards arrow on top equals U with rightwards arrow on top minus T with rightwards arrow on top
space space space space space equals space a with rightwards arrow on top plus lambda b with rightwards arrow on top minus open parentheses a with rightwards arrow on top minus b with rightwards arrow on top close parentheses
space space space space space equals left parenthesis lambda space minus 1 right parenthesis b with rightwards arrow on top
    For the points to be collinear, the vectors should be collinear too. Two vectors are collinear if they are scalar multiple of each other. Let the scalar multiple be p.
    stack S T with rightwards arrow on top equals p stack T U space with rightwards arrow on top
minus 2 b with rightwards arrow on top equals p left parenthesis lambda space minus 1 right parenthesis b with rightwards arrow on top
space minus 2 space equals p left parenthesis lambda space minus space 1 right parenthesis
space space space space space p space equals fraction numerator negative 2 over denominator left parenthesis lambda space minus space 1 right parenthesis end fraction
    So, λ will have all real values.
    The points are collinear for all real values of λ.

    For such questions, we should know how to find the vector joining two points. Also, we should know the condition for two vectors to be collinear.

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