Hi, heres the question:
The curve with equation y=f(x) is such that
dy/dx = 3x^2+4x+k
where k is a constant
Given that C passes through the points (0,-2) and (2,18).
a) Show that k=2 and find an equation for C
b) Show that the line with equation y=x-2 is a tangent to C and find the coordinates of the point of contact.
I can do a)
k=2 c=-2 y=x^3+2x^2+2x-2
but with b, the answer says:
x^3+2x^2+2x-2=x-2
x^3+2x^2+x=0
x(x^2+2x+1)=0
x(x+1)^2=0
REPEATED ROOT THEREFORE TANGENT
point of contact where x=-1
THEREFORE (-1,-3)
can someone please explain where they got those last three lines from.
Cheers guys!