How do you write #(sec theta - 1)(sec theta + 1)# in terms of sine and cosine?

2 Answers
Mar 6, 2018

Kindly refer to the Explanation.

Explanation:

Using the Identity : #sec^2theta=tan^2theta+1#, we have,

#(sectheta-1)(sectheta+1)=sec^2theta-1=sin^2theta/cos^2theta#.

Otherwise,

#(sectheta-1)(sectheta+1)#,

#=sec^2theta-1#,

#=1/cos^2theta-1#,

#=(1-cos^2theta)/cos^2theta#,

#=sin^2theta/cos^2theta#, as above!

Mar 6, 2018

The expression in terms of sine and cosine is #sin^2theta/cos^2theta#.

Explanation:

First, you should multiply the expression and simplify as far as you can. Then, write everything in terms of sine and cosine.

Here are the identities we'll use:

#color(white){color(black)( (sectheta=1/costheta, qquadqquad(1.1)), (sec^2theta=1/cos^2theta, qquadqquad(1.2)), (tantheta=sintheta/costheta, qquadqquad(2.1)), (tan^2theta=sin^2theta/cos^2theta, qquadqquad(2.2)), (sin^2theta+cos^2theta=1, qquadqquad(3.1)), (sin^2theta/cos^2theta+cos^2theta/cos^2theta=1/cos^2theta, qquadqquad(3.2)), (tan^2theta+1=sec^2theta, qquadqquad(3.3)):}#

Some notes: identity #(1.2)# was achieved by squaring both sides of identity #(1.1)# (same with #(2.2)# and #(2.1)#).

Similarly, identity #(3.2)# was achieved by dividing all the terms in identity #(3.1)# by #cos^2theta#. Then, identity #(3.3)# was reached by simplifying identity #(3.2)# using previously-proved identities #(1.2)# and #(2.2)#

Now, here's the expression:

#color(white){color(black)( ((sectheta-1)(sectheta+1), qquadqquad"The problem"), (sec^2theta+sectheta-sectheta-1, qquadqquad"Multiplying out the expression"), (sec^2thetacolor(red)cancelcolor(black)(+sectheta-sectheta)-1, qquadqquad"Like terms cancel out"), (sec^2theta-1, qquadqquad"Rewrite the above step"), (tan^2theta+1-1, qquadqquad"Replace "sec^2theta" with "tan^2theta+1" using identity "(3.3)), (tan^2thetacolor(red)cancelcolor(black)(+1-1), qquadqquad"Like terms cancel out"), (tan^2theta, qquadqquad "Rewrite the above step"), (sin^2theta/cos^2theta, qquadqquad "Replace "tan^2theta" with "sin^2theta/cos^2theta" using identity "(2.2)):}#

That's the answer. Hope this helped!