9514 1404 393
Explanation:
Add sin(θ)
cos(θ) = sin(θ)
Divide by cos(θ)
1 = sin(θ)/cos(θ) = tan(θ)
Square both sides
1 = tan²(θ)
Add 1
2 = tan²(θ) = sec²(θ)
Take the square root
√2 = sec(θ) . . . . for 0 < θ < π
_____
Additional comment
The equation cos(θ) -sin(θ) = 0 has two solutions: θ = π/4 and θ = 5π/4. For the first-quadrant solution, sec(θ) = √2. For the third-quadrant solution, sec(θ) = -√2.
