y = sinx + cosx = √2sin(x + π/4)y' = √2cos(x + π/4),y'' = -√2sin(x + π/4)y' = 0 => cos(x + π/4) = 0x + π/4 = 2kπ + π/2 或 x + π/4 = 2kπ - π/2x = 2kπ + π/4 或 x = 2kπ - 3π/4y''|(2kπ +...