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1)

 In ABC, with usual  notations  , if   cosA=sinBsinC then the triangle  is ........


A) Acute angled triangle

B) Equilateral triangle

C) Obtuse angled triangle

D) Right angled triangle

Answer:

Option D

Explanation:

 Key Idea  Use sine rule,

    sinAa=sinBb=sinCc

 we have,

   cosA=sinBsinC

 b2+c2a22bc=bc(sinAa=sinBb=sinCc=k)

    b2+c2a2=2b2

    c2a2=b2

     c2=a2+b2

     ABC  right angled  at  C