Question

Let cos x =1√10 with x in QIV and find: cot2x

Answer 1

cos x = 1/√10
sin x = √( 1 - 1 /10 ) = √9/10 = - 3/√10 ( because x is in Q IV )
cot 2 x = cos 2 x/sin 2x = ( cos²x - sin²x)/ (2 sinx cosx ) =
= ( 1/10 - 3/10 )/(2 * (-3/√10)*1/√10)= (- 8/10 ) / (-6/10 ) = 8/6 = 4/3