y'={1/[x+√(x²+a²)]}*[x+√(x²+a²)]'={1/[x+√(x²+a²)]}*[1+1/2√(x²+a²)*√(x²+a²)']={1/[x+√(x²+a²)]}*[1+2x/2√(x²+a²)]={1/[x+√(x²+a²)]}*{[x+√(x²+a²)]/√(x²+a²)}=1/√(x²+a²)
