y=x/(1+x²)y ′ = {1+x²-x*2x}/(1+x²)² = (1-x²)/(1+x²)²y ′′ = {-2x(1+x²)² - (1-x²)*2(1+x²)*2x} /(1+x²)^4= {-2x(1+x²) - (1-x²)*2*2x} /(1+x²)³= -2x{(1+x²) + (1-x²)*2} /(1+x²)³= -2x{3-x²} /(1+x²)³
