Answer:
√10 / 10
Explanation:
tan θ > 0 and sin θ < 0, so θ is in quadrant III. That means cos θ < 0.
cos(θ + π/4)
Use angle sum formula.
cos θ cos(π/4) − sin θ sin(π/4)
½√2 cos θ − ½√2 sin θ
Factor.
½√2 cos θ (1 − tan θ)
½√2 cos θ (1 − 2)
-½√2 cos θ
Write in terms of secant.
-½√2 / sec θ
Use Pythagorean identity (remember that cos θ < 0).
-½√2 / -√(1 + tan²θ)
-½√2 / -√(1 + 2²)
½√2 / √5
√10 / 10