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Integrals

Question
CBSEENMA12036040

The number of points having both coordinates as integers that lie in the interior of the triangle with vertices (0,0), (0,41) and (41,0) is

  • 901

  • 861

  • 820

  • 780

Solution

D.

780

Required point (x,y) is such  that it satisfies
 x +y < 41
and x> 0 and y>0
Number of positive integral solutions of the equation x +y+ k = 41 will be number of intergral coordinates in the bounded region.

therefore, the total number of integral coordinates,
 = straight C presuperscript 41 minus 1 end presuperscript subscript 3 minus 1 end subscript space equals space straight C presuperscript 40 subscript 2 space equals space fraction numerator 40 space factorial over denominator 2 space factorial space 38 space factorial end fraction space equals space 780

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