Move the slider or input values for [i]a[/i]. The blue curve is the graph of [i]y=sin(x)[/i], and the red line is the tangent to the curve at the point [i](a, sin(a))[/i]. Recall that the slope of the tangent line to the curve at [i](a,f(a))[/i] is the value [i]f'(a)[/i]. The red point is the point [i](a, f'(a))[/i]. Notice that the graph of [i]y=d/dx(sin(x))[/i] is [i]cos(x)[/i].