Use the fact that the length of an arc intercepted by an angle is proportional to the radius to find the area of the sector given r = 3 cm and θ =
π
4
.

Sector area = (Central Angle (Degrees) / 360) * PI * radius^2
One way to solve this is that an angle of (PI/4) = 45 degrees.
45 degrees is one eighth of a circle.
Area of ENTIRE circle = PI*radius^2 = PI * 3^2 = 28.2743338823  square centimeters
Area of the sector is one eighth of this =  3.5342917353 square centimeters