ds=r^2du/2S/8=∫[0,π/4]4a^2(cosu)^2du/2=a^2∫[0,π/4](1+cos2u)du=a^2*π/4+(a^2/2)∫[0,π/4]dsin2u =πa^2/4+a^2/2S=2πa^2+4a^2
555
