Limits · Continuity · Differentiability
PromiseFull marks here mean: one-sided limits table → continuity vs differentiability as separate verdicts → simplify before L’Hôpital → never apply L’Hôpital without 0/0 or ∞/∞.
01. Limit exists iff LHL=RHL. Continuity needs limit = f(a). Differentiability needs LHD=RHD (finite). Differentiable ⇒ continuous; converse false (|x| at 0).
02. Standard limits (sinx/x, etc.) and algebraic cancellation beat illegal L’Hôpital. State the indeterminate form explicitly when you use L’Hôpital.
03. Rolle/MVT: check hypotheses (continuity on closed, differentiability on open, f(a)=f(b) for Rolle) before concluding existence of c.
How Advanced asks this
- Continuity/differentiability of piecewise f at a join
- Floor / absolute-value behaviour on an interval
- Limits needing rationalisation or standard forms
- MC on Rolle/MVT hypotheses
- NUM value of a limit
Traps that leak marks
- L’Hôpital without indeterminate form
- Claiming differentiability at |x| kink
- Ignoring endpoints on closed intervals
- Mixing continuity of f with continuity of f′
- L’Hôpital on a determinate quotient
- Claiming differentiability from two-sided limit of f alone
- Applying MVT without verifying continuity on [a,b] and differentiability on (a,b)
Must cover (this topic)
- L’Hôpital and continuity of composites
60–90s process
- 0–15s — Mark type: limit / continuity / differentiability / MVT. List candidate points.
- 15–35s — For continuity/diff: write LHL, RHL, f(a); then LHD, RHD if needed.
- 35–75s — Evaluate. Use standard limits, rationalisation, or L’Hôpital after stating form.
- 75–90s — Check endpoints on closed intervals; do not claim differentiability without continuity logic where relevant.
Methods that change the score
This chapter decides whether you trust your Calculus. These methods force the LHL/RHL table and ban illegal L’Hôpital — exactly what Advanced tests.
Method 01 · Table method
Makes continuity vs differentiability feel like bookkeeping, not philosophy.
Do this: LHL/RHL/f(a)/LHD/RHD in a tiny table at each break.
Method 02 · Simplify then L’Hôpital
Protects you from illegal L’Hôpital — Advanced designs those traps on purpose.
Do this: Cancel factors / use conjugates / standard limits first.
Method 03 · Hypothesis checklist
Rolle/MVT MC becomes a tick list — conclusions without hypotheses get rejected.
Do this: For Rolle/MVT MC, tick each hypothesis true/false.
Method 04 · Indeterminate stamp
Forces you to say 0/0 or ∞/∞ out loud before differentiating a quotient.
Do this: Write ‘0/0’ or ‘∞/∞’ before L’Hôpital.
Quick formulas
- Limit existence: lim x→a f = L iff LHL = RHL = L — Two-sided limits
- Continuity at a: f(a) defined, lim exists, lim = f(a) — Continuity checks
- Derivative definition: f′(a)=lim h→0 [f(a+h)−f(a)]/h — First principles / one-sided derivatives
- Standard limit: lim θ→0 sinθ/θ = 1 (radians) — Trig limits
- L’Hôpital: If 0/0 or ∞/∞, lim f/g = lim f′/g′ (when latter exists) — Indeterminate quotients only
- Rolle: f cont [a,b], diff (a,b), f(a)=f(b) ⇒ ∃c f′(c)=0 — Existence of horizontal tangent
- Lagrange MVT: f cont [a,b], diff (a,b) ⇒ ∃c f′(c)=[f(b)−f(a)]/(b−a) — Mean slope realisation
- Squeeze theorem: g≤f≤h and lim g=lim h=L ⇒ lim f=L — Oscillatory / boundable limits
Common question families
Piecewise continuity / differentiability
Looks like: f defined differently on intervals; ask at join / floor / |x|
Fast model: Tiny table: LHL, RHL, f(a), then LHD, RHD — separate verdicts.
0/0 limit
Looks like: Quotient or difference that vanishes at the point
Fast model: Simplify / rationalise / standard limits FIRST; L’Hôpital only after stamping 0/0 or ∞/∞.
Rolle / MVT hypothesis MC
Looks like: Which theorem applies / existence of c
Fast model: Tick hypotheses one-by-one; conclusion only if all hold.
∞/∞ or e-limit / standard forms
Looks like: lim (1+u)^{1/u}; lim (a^x−1)/x; forms that become 1^∞ after rewrite
Fast model: Rewrite to exp(lim ln) or standard (e^x−1)/x → 1; stamp the form first.
Sample preview · 0/0 limit
Evaluate lim x→0 (√(1+x)−1)/x.
Answer: 1/2
Simplify first; L’Hôpital only after stamping 0/0.