math(advanced algebra&trigonometry)
posted by Rony on .
the expression cos4x cos3x + sin4x sin3x is equivalent to
which one of the following is the answer:
1) sinx
2) sin7x
3) cosx
4) cos7x

Look up the equation for cos(xy) and it should be obvious what the answer is.
Think of x as 4x and y as 3x.
http://www.csgnetwork.com/trigtriformulatables.html 
it is cosx
thanks 
Seca=4x + 1/x than secb + tana what please tell me after 2days