Question

how to tell if linear approximation is over or under?

Answer 1

If f (t) > 0 for all t in I, then f is concave up on I, so L(x0) < f(x0), so your approximation is an under-estimate. If f (t) < 0 for all t in I, then f is concave down on I, so L(x0) > f(x0), so your approximation is an over-estimate.