To find the maximum of f(x) = sin(x) + cos(x)
find the first derivative: f-1(x) = cos(x) - sin(x)
and set this expression equal to zero: cos(x) - sin(x) = 0
cos(x) = sin(x)
1 = sin(x) / cos(x)
1 = tan(x)
But, tan(x) = 1 at 45o.
sin(45o) + cos(45o) = sqrt(2)/2 + sqrt(2)/2 = sqrt(2)