Question #2d450

1 Answer
P dilip_k
Nov 28, 2017

#cosA+cos17A#

#=2cos9Acos8A#

#=2cos90cos80#

#=2*0*cos80#

#=0#

So #cosA+cos17A=0#
Similarly

  • #cos2A+cos16A=0#

  • #cos3A+cos15A=0#

  • #cos4A+cos14A=0#

  • #cos5A+cos13A=0#

  • #cos6A+cos12A=0#

  • #cos7A+cos11A=0#

  • #cos8A+cos10A=0#

Again

  • #cos9A=cos90=0#

and

  • #cos18A=cos180=-1#

Adding all these equations we get

#cosA+cos2A+cos3A +.....+cos18A =-1#

Related questions
Impact of this question
248 views around the world
You can reuse this answer
Creative Commons License