The value of sin−1[cos(33π5)] is
(a) 3π5 (b) −7π5 (c) π10 (d) −π10
(d) We have,
sin−1(cos33π5)=sin−1[cos(6π+3π5)]=sin−1[cos(3π5)] [∵ cos(2nπ + θ)=cos θ]=sin−1[cos(π2+π10)]=sin−1(−sinπ10)=−sin−1(sinπ10) [∵ sin−1(−x)=−sin−1 x]=−π10 [∵ sin−1 (sin x)=x, x∈(−π2,π2)]