Verify the identity cos29 * cos1 - sin29 * sin1 = cos 28 * cos 2 + sin 28 * sin 2.

We notice that the given sum is the expanding of the
followings:

cos (a - b) = cos a*cos b + sin a*sin
b

and

cos (a + b) = cos a*cos
b - sin a*sin b

If we'll substitute a and b by 28 and 2,
we'll get the given sum, that has to be calculated:

cos
(28-2) = cos 28 * cos 2 + sin 28 * sin 2

cos
28 * cos 2 + sin 28 * sin 2 = cos 26

Now,
we'll substitute  a and b by 29 and 1, to calculate the
difference:

cos (29+1) = cos29 * cos1 - sin29 *
sin1

cos29 * cos1 - sin29 * sin1 = cos
30

It is obvious that the result of the
difference is not the same with the result of the
sum:

cos 30 is not equal to cos
26!

Note: In case of identity between the
given expression we'll have to make the correction in the expression cos 28 * cos 2 +
sin 28 * sin 2.

This expression has to be a difference
instead of a sum:

cos 28 * cos 2 - sin 28 * sin 2 = cos
(28+2) = cos 30