why here it says affine functions are both convex and concave?

nyu.edu/access/content/group/dd0f411-0121-4818-827B-11613bafe133/week 2003/3 curse dimensionality.pdf NU Classes: Ho - NYU Shangha ARCC) Problems te 12 / 20 Coursera Online : (16666 ty.pdf 118% 34 / 59 Example of convex functions a+b PE Exponential: f(x) = eax for Va ER Affine functions: f(x) = a' x + b where x ER" y Quadratic functions: f(x) = }x"Ax+b"x + c for a symmetric matrix A e rnxn and be R" and c ER Nonnegative weighted sums of convex functions f(x) = {{-1 wifi(x) Where fi.f2.... fx are convex functions and W1, W2, ...Wk are nonnegative real numbers Affine functions are both convex and concave