Answer:
.
Explanation:
Angle sum identity for cosine:
.
Pythagorean identity:
for all real
.
Subtract
from both sides of the Pythagorean identity to obtain:
.
Apply angle sum identity to rewrite
.
.
follows from the Pythagorean identity. Hence, it would be possible to replace the
in the previous expression with
.
.
Conclusion: