Posted by **jessika** on Thursday, November 15, 2007 at 7:31pm.

i have a few questions

cosx + cosx =2secx

----- -----

1+sinx 1-sinx

cos(x-B)-cos(x-B) = 2sinxsinB

csc2x= 1 secx cscx

---

2

cotx= cos5x+cos3x

-----------

sin 5x-sin 3x

- math -
**Reiny**, Thursday, November 15, 2007 at 8:09pm
for the first one:

LS = cosx/(1+sinx) + cosx/(1-sinx)

getting a common denominator of (1+sinx)(1-sinx)

= [cosx(1-sinx) + cosx(1+sinx)]/(1+sinx)(1-sinx)

= 2cosx/(1-sin^2)

= 2cosx/cos^2x

= 2/cosx

= 2secx

= RS

for cos(x-B)-cos(x-B) = 2sinxsinB

use the formula cos(A-B) = cosAcosB + sinAsinB on the Left Side, it comes apart very nicely after that

For the last two, try changing all trig ratios into sines and cosines.

Show me what you get

- math -
**Reiny**, Thursday, November 15, 2007 at 8:19pm
for cos(x-B)-cos(x-B) = 2sinxsinB

you must have a typo, the LS is zero the way it stands

I am sure you meant

cos(x-B)-cos(x**+**B) = 2sinxsinB

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