Could I get some help with the following:

sin{1/2acrsin(-7/25)}

cos(1/2arctan 8/15)

tan(1/2acrcos 3/5)

I know how to do them if the 1/2 isn't in there but I don't know how to get the answers for them. The answers btw are:

1. -1/10 * 2^(1/2)

2. 4/17 * 17^(1/2)

3. 1/2

Thanks in advance.