(1- cosa) (1+(03a)

It seems like there's a small typo in your expression. Assuming you meant to write \( (1 - \cos a)(1 + \cos a) \), I can help simplify that.

The expression can be simplified using the difference of squares formula:

\[ (1 - \cos a)(1 + \cos a) = 1^2 - (\cos a)^2 = 1 - \cos^2 a \]

According to the Pythagorean identity, we know that:

\[ \sin^2 a + \cos^2 a = 1 \]

Thus, we can express \( 1 - \cos^2 a \) in terms of sine:

\[ 1 - \cos^2 a = \sin^2 a \]

Therefore, the simplified form of the expression is:

\[ (1 - \cos a)(1 + \cos a) = \sin^2 a \]

If this is not what you were looking for or if the original expression was different, please provide clarification.