Step 1: Formula / Definition}
\[
\text{Area} = \text{Area of quarter circle} - \text{Area of triangle}
\]
Step 2: Calculation / Simplification}
Circle: \(x^2 + y^2 = 1\). Line: \(x + y = 1\)
Intersection: \(x^2 + (1-x)^2 = 1 \Rightarrow 2x^2 - 2x = 0 \Rightarrow x = 0, 1\)
Points: \((0,1)\) and \((1,0)\)
Area of sector = \(\frac{\pi}{4}\)
Area of triangle = \(\frac{1}{2} \times 1 \times 1 = \frac{1}{2}\)
Area = \(\frac{\pi}{4} - \frac{1}{2}\)
Step 3: Final Answer
\[
\frac{\pi}{4} - \frac{1}{2}
\]