Hi everyone. Can someone please explain these questions?

1) Find the exact value of cos 105 degrees

how would i go about doing that? it's not just a simple calculator problem, but rather one probably involving 30-60-90 or 45-45-90 triangles. the answer has radicals in it.

2) A surveyor determines that the angle of elevation of a mountain peak is 35 degrees. Moving 1000 feet further away from the mountain, the surveyor determines the angle of elevation to be 30 degrees. Find the mountain's height.

Thanks!