4.1.1 · D5Calculus I — Limits & Derivatives
Question bank — Intuitive concept of a limit — table of values, graphical
True or false — justify
can be true even if is undefined.
True — the limit only inspects near , never itself, so a hole at leaves the approached value untouched.
If then automatically .
False — knowing the value at the point tells you nothing about the approach; a stray dot can sit at while the curve heads elsewhere. (Agreement of the two is exactly Continuity at a point.)
If both one-sided limits exist, the two-sided limit exists.
False — they must also be equal; existing separately isn't enough, as a jump shows left , right , both exist, yet disagree.
A table showing proves the limit is exactly .
False as a proof — a table only builds evidence for a forecast; it can never guarantee the trend, since a function could veer at values smaller than any you tabulated.
"The limit is " and " reaches " mean the same thing.
False — a limit is the target of an endless squeeze; the outputs may approach forever without any landing on it.
If does not exist, then must be undefined.
False — can be perfectly defined at a jump; the limit fails because the two sides disagree, not because the point is missing.
Changing the single value can change .
False — the limit ignores the lone point entirely, so editing that one value moves nothing about the approach.
Two different functions that agree everywhere except at have the same limit at .
True — the limit reads only the surrounding values, which are identical, so the disagreement at is invisible to it.
Spot the error
" undefined, so no limit exists."
The error is treating as a final answer; it's an indeterminate form signalling simplify first — factor and cancel to get .
"We cancel to get , so the two functions and are identical."
They agree only for ; the original is undefined at while isn't. The cancellation is legal inside the limit precisely because keeps .
"I computed the left-hand limit is , therefore ."
One side is not enough — the right-hand limit could be anything. You must confirm both sides and that they agree before naming the two-sided limit.
"Direct substitution gave , so substitution always finds the limit."
Substitution is only a first guess that happens to work for nice (continuous) functions; at holes, jumps, or traps it gives undefined or the wrong value.
"The values approach but never equal it, so really the limit is 'almost ', not ."
There is no number "almost "; the limit is defined as the single value the outputs get arbitrarily close to, and that value is exactly .
" has because ."
That simplification only holds for ; for it equals . Left limit right limit , so the two-sided limit does not exist.
"Since the graph has a hole at , the function has no limit there."
A hole removes only the point , not the approach toward height from both sides — so the limit is despite the hole.
Why questions
Why must we forbid in the definition rather than just include it?
Because the whole purpose is to describe well-behaved approach at broken spots; allowing would re-import the very division-by-zero or undefined value we're trying to step around.
Why do we insist both one-sided limits agree instead of just picking one?
A limit names a single destination; if the two approach paths head to different heights there is no one destination, so calling either the "limit" would be arbitrary. (See One-sided limits and limits at infinity.)
Why is a table a forecast and not a proof?
A table samples finitely many -values; the true guarantee of "arbitrarily close" needs the Formal epsilon-delta definition of a limit, which handles every nearness at once, not just the rows you wrote.
Why does factoring help at a point?
The comes from a shared factor vanishing on top and bottom; cancelling it exposes the finite value the ratio was really heading toward once the fake cancellation is removed.
Why does the limit "ignore" the stray dot in ?
The limit reads the trend of nearby outputs (all near ); the lone value at is not "near" any other point, so it never enters the squeeze that defines the limit.
Why is called indeterminate rather than just "undefined"?
"Undefined" means no value; "indeterminate" means the form alone doesn't decide the answer — different expressions can limit to different numbers, so more work is required.
Why does the derivative need this exact idea of a limit?
The slope of a curve is a ratio of a vanishing rise over a vanishing run at a single point; only a limit lets us name the value that ratio approaches. (This is The derivative as a limit of difference quotients.)
Edge cases
Does exist?
No — as the input races through all values, so the output oscillates endlessly between and and never settles on one destination.
If grows without bound as , do we say the limit "exists"?
No — "the limit is " describes the behaviour (unbounded growth) but no finite value is approached, so a finite limit does not exist there.
Can a two-sided limit exist at a point where isn't even defined nearby on one side (like an endpoint)?
Not as a two-sided limit — you can only take the one-sided limit from the side where the function lives; the missing side has nothing to approach. (See One-sided limits and limits at infinity.)
If near but , is the limit trapped?
Not enough — bounding between two different constants leaves room to wander. You need bounds that both approach the same value, which is the Squeeze (Sandwich) Theorem.
For a jump function, do the one-sided limits still exist even though the two-sided one doesn't?
Yes — each side individually homes in on its own height, so both one-sided limits exist; the two-sided limit fails only because those two heights differ.
Is for the constant zero function, hole or not?
Yes — every nearby output is , so the approached value is regardless of what happens at ; constant functions approach their constant everywhere.
At an isolated point (function defined only at and nowhere near it), what is ?
It's not defined — with no other points to approach through, there is no trend to read; the limit concept simply doesn't apply.
Active Recall
Recall The single sentence that resolves most traps here
A limit watches the approach to through nearby points, never the value at ; so verdicts hinge on "what are both sides heading toward?" — not on .
Connections
- One-sided limits and limits at infinity
- Formal epsilon-delta definition of a limit
- Continuity at a point
- Indeterminate forms and algebraic simplification
- The derivative as a limit of difference quotients
- Squeeze (Sandwich) Theorem