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 DExplanation: Key Idea Use sine rule, sinAa=sinBb=sinCc we have, cosA=sinBsinC ⇒b2+c2−a22bc=bc(∵sinAa=sinBb=sinCc=k) ⇒ b2+c2−a2=2b2 ⇒ c2−a2=b2 ⇒ c2=a2+b2 ⇒ △ ABC right angled at ∠C