Question

10 points!!! please help :(

Answer 1

To complete the identity, we need these fundamental identities:

$1)\displaystyle{sec(x)=\frac{1}{cos(x)}$

$2) cos(x-y)=cos(x)cos(y)+sin(x)sin(y)$

$\displaystyle{csc(x)= \frac{1}{sin(x)}$


Thus, by identity 1 we have:

$\displaystyle{ sec( \frac{ \pi }{2}-\theta )= \frac{1}{cos(\frac{ \pi }{2}-\theta)}$

by identity :

$\displaystyle{cos(\frac{ \pi }{2}-\theta)=cos(\frac{ \pi }{2})cos(\theta)+sin(\frac{ \pi }{2})sin(\theta)$

recall the values :

$\displaystyle{ sin(\frac{ \pi }{2})^R=sin(90^o)=1$

$\displaystyle{ cos(\frac{ \pi }{2})^R=cos(90^o)=0$, 


so: 

$cos(\frac{ \pi }{2})cos(\theta)+sin(\frac{ \pi }{2})sin(\theta)=0+sin(\theta)=sin(\theta)$


Putting all these together, we have:


$\displaystyle{ sec( \frac{ \pi }{2}-\theta )= \frac{1}{cos(\frac{ \pi }{2}-\theta)}= \frac{1}{cos(\frac{ \pi }{2})cos(\theta)+sin(\frac{ \pi }{2})sin(\theta)}= \frac{1}{sin(\theta)}}$

which is equal to $csc(\theta)$, by identity 3


Answer: D