posted by Daniel .
Find an equation of the tangent line to the curve at
y = cosx - sinx (pi, -1)
I found y'= -sinx - cosx
and used the equation of the tangent
y = mx +b
I input the values and get -1 - [(-sinx - cosx) * pi] = b
the answer says it is y = x - 1 - pi
I have no idea how the answer got that.