Maths-
General
Easy

Question

If ABC is a triangle and tan space A over 2 comma tan space B over 2 comma tan space C over 2 are H.P then the minimum value of  cot space B over 2 is equal to

  1. negative square root of 3
  2. square root of 3
  3. 2
  4. -2

The correct answer is: square root of 3


    tan space A over 2 comma tan space B over 2 comma tan space C over 2 space a r e space i n space H. P
therefore space tan B over 2
equals fraction numerator 2 tan begin display style A over 2 end style tan begin display style C over 2 end style over denominator tan begin display style A over 2 end style plus tan begin display style C over 2 end style end fraction
equals fraction numerator 2 begin display style fraction numerator sin begin display style A over 2 end style over denominator cos begin display style A over 2 end style end fraction end style times begin display style fraction numerator sin begin display style C over 2 end style over denominator cos begin display style C over 2 end style end fraction end style over denominator begin display style fraction numerator sin begin display style A over 2 end style over denominator cos begin display style A over 2 end style end fraction end style plus begin display style fraction numerator sin begin display style C over 2 end style over denominator cos begin display style C over 2 end style end fraction end style end fraction
equals fraction numerator 2 sin begin display style A over 2 end style cos begin display style C over 2 end style over denominator sin begin display style A over 2 end style cos begin display style C over 2 end style plus sin begin display style C over 2 end style cos begin display style A over 2 end style end fraction
equals fraction numerator cos open parentheses begin display style A over 2 end style minus begin display style C over 2 end style close parentheses minus cos open parentheses begin display style A over 2 end style plus begin display style C over 2 end style close parentheses over denominator sin open parentheses begin display style A over 2 end style plus begin display style C over 2 end style close parentheses end fraction
equals fraction numerator cos open parentheses A over 2 minus C over 2 close parentheses minus cos open parentheses begin display style pi over 2 end style minus begin display style B over 2 end style close parentheses over denominator sin open parentheses begin display style pi over 2 end style minus begin display style B over 2 end style close parentheses end fraction
tan B over 2 equals fraction numerator cos open parentheses A over 2 minus C over 2 close parentheses minus sin open parentheses begin display style B over 2 end style close parentheses over denominator cos open parentheses begin display style B over 2 end style close parentheses end fraction
fraction numerator sin B over 2 over denominator cos begin display style B over 2 end style end fraction equals fraction numerator cos open parentheses A over 2 minus C over 2 close parentheses minus sin open parentheses begin display style B over 2 end style close parentheses over denominator cos open parentheses begin display style B over 2 end style close parentheses end fraction
sin B over 2 equals cos open parentheses A over 2 minus C over 2 close parentheses minus sin open parentheses B over 2 close parentheses
2 sin B over 2 equals cos open parentheses A over 2 minus C over 2 close parentheses
therefore c o t B over 2 equals square root of 3

    A sequence of numbers is called a harmonic progression if the reciprocal of the terms are in AP. In simple terms, a,b,c are in HP if 1/a, 1/b, 1/c are in AP.

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