两点所在直线段的方程是y = 2x - 1.dy = 2 dx
∫L 2x dx + (y - x) dy
= ∫(1→2) {2x + [(2x - 1) - x](2)} dx
= ∫(1→2) (4x - 2) dx
= [ 4(x²/2) - 2x ] |(1→2)
= [ 2(2)² - 2(2) ] - [ 2 - 2 ]
= 4