Maths-
General
Easy

Question

Two cars A and B are moving uniformly on two straight roads at right angles to one another at 40 and 20 km/hr respectively. A passes the intersection of the road when B has still to move 50 km to reach it. The shortest distance between the two cars and the time when they are closest are

  1. 20 square root of 5 k m comma 30 minutes    
  2. 20 km, 10 minutes    
  3. 20 km, 20 minutes    
  4. None of these    

The correct answer is: 20 square root of 5 k m comma 30 minutes


    Let the two be closest after time t, their initial position are O for A and B such that OB = 50. If they are now at A to the power of ´ end exponent and B to the power of ´ end exponent respectively, then
    O A to the power of ´ end exponent equals 40 t comma O B to the power of ´ end exponent equals O B minus B B to the power of ´ end exponent equals 50 minus 20 t
    A to the power of ´ end exponent rightwards arrow over short leftwards arrow B to the power of ´ end exponent equals left square bracket left parenthesis 40 t right parenthesis to the power of 2 end exponent plus left parenthesis 50 minus 20 t right parenthesis to the power of 2 end exponent right square bracket to the power of 1 divided by 2 end exponent
    If A to the power of ´ end exponent B to the power of ´ end exponent is least then
    Z equals A to the power of ´ end exponent B to the power of ´ 2 end exponent equals left parenthesis 40 t right parenthesis to the power of 2 end exponent plus left parenthesis 50 minus 20 t right parenthesis to the power of 2 end exponent is also least.
    \ fraction numerator d z over denominator d t end fraction equals 2 left parenthesis 40 t right parenthesis 40 plus 2 left parenthesis 50 minus 20 t right parenthesis left parenthesis negative 20 right parenthesis equals 0
    Þ 80 t minus 50 plus 20 t equals 0
    Þ t equals fraction numerator 1 over denominator 2 end fraction h r equals 30minutes
    Clearly fraction numerator d to the power of 2 end exponent z over denominator d t to the power of 2 end exponent end fraction equals 100 equals plus v e
    \ z is least
    Putting t equals fraction numerator 1 over denominator 2 end fractionwe get z = 202+ 402 = 202×5 = A to the power of ´ end exponent B to the power of ´ 2 end exponent
    \A ´ B ´ equals 20 square root of 5 equals 44.8 m

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