trig help (sin cos tan)

[HoangDDS]

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which of the following is true for all values of x such that 0 < x < pi/2

a) sin x = cos^2 + 1
b) tan x = (sin x)(cos x)
c) tan x = (cos x)/(sin x)
d) sin^2 x =1 - cos^2x
e) sin x = 1/ cos x

Answer is D. But how do I start this question? What should I know before I do this question? Any comments or help is appreciated.

 

[Mstoothlady2012]

you need to know that sin^2X + cos ^2X = 1, after rearranging you would get sin^2 x = 1 - cos^2 X

 

[HoangDDS]

ohh duh! that was simple. Thank you!

 

[HoangDDS]

i guess i was trying to make this question way harder than it needs to be. I was substituting in 45 degree but it got really messy. Thanks tho.

 

[Mstoothlady2012]

🙂

 

[klutzy1987]

the way i did this one was first to divide 180/2. That left 0<x<90. From there i just looked at which of the trig rules they had up there were correct. The only rule thats correct is sin squared + cos squared =1. Rewriting that gives you D.