y'= 1/√(2x)
y'|(x=2) = 1/2
切线:y=1/2(x-2) +2 = 1/2 x +1
x = 2y -2
S=∫ [y²/2 - (2y -2)] dy
= [ y³/6 - y² + 2y]
= 4/3