Find the angle θ (if it exists) in the interval [0°, 90°) for which sin θ = cos θ.

Sine = opposite / hypotenuse
cosine = adjacent / hypotenuse
So, in a right triangle which has the opposite side equal to the adjacent side, we will have the sin (theta) = cosine (theta)
This would be an isosceles right triangle which is also called a 45 45 90 triangle. 
sine (45) = 0.70711 and the
cosine (45) = 0.70711