6. Graph y=sinx + cosx restricted to 0<=x<=pi/2. Locate the points on the graph at which the tangent line appears to have slope zero. Then find those x, 0<=x<=pi/2 , for which the function f(x)=sinx+cosx has a horizontal tangent line. (You need to solve the equation: f' (x)=0 for 9<x< pi/2 )
Derivatives and tangent lines are investigated.