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C3 trig help please

1)Given that secA = - 3, where 90 < A < 180,

calculate the exact value of tan A

This is what I did;

sec^2A = 9
1 + tan^2A = 9
tan^2A = 8
tanA = +-Root8
tanA = +-2root2

However, the book says the answer is -2root2, why is it not + and - ???

2)Given that sec x = k , |k|>1, and that x is obtuse, express in terms of k:

a) cos x b) tan^2x c) cot x d)cosec x

For part a I got ~ 1/k ~ which is correct
For part b I got ~ k^2 - 1 ~ which is correct
For part c I got ~ 1/Root(k^2 - 1) ~ which is not correct, the answer says ~ -1/Root(k^2 - 1)
For pat d I got ~ k/Root(k^2 - 1) ~ which is not correct, the answer says ~ - k/Root(k^2 - 1)

Why are the answers for part c and d negative ???