JentooSAT®

SAT® · Math: Advanced Math

Interpreting a Constant, Variable, Factor, or Term in Context

Every number in a model formula stands for something specific in the story it describes — a starting amount, a rate of change, or a per-step multiplier. Reading the formula means matching each piece to its job.

A model formula is a sentence written in symbols. Every piece — the constant sitting alone, the coefficient attached to a variable, the base of an exponential, the number squared in a quadratic — translates back into a phrase about the real situation. Your job is to find that phrase. Ask: if I plug in t = 0, what does this number mean physically? What happens to the output when t increases by 1? Those two questions usually unlock the whole formula.

In an exponential like a(b)t, a is the starting value, the amount when t = 0, because b0 = 1 and everything collapses to a. The base b is the per-step multiplier: whatever amount you have, multiply by b to get next year's amount. If b is between 0 and 1, the quantity shrinks; if b is greater than 1, it grows. In a quadratic like h(t) = -16t2 + vt + h0, h0 is the starting height, v is the initial speed, and -16 comes from gravity's pull, fixed by the units (feet and seconds) — it is not something the scenario gets to choose.

This skill sits right behind building the formula in the first place: once you can set up a quadratic or exponential model, you need to be able to read it back out, because test questions often give you the formula fully formed and ask what a piece of it means, or ask you to rewrite it so a different piece becomes visible, like switching an exponential to a different base to expose a different time step. Every rewriting trick you learn for these forms exists to make some constant's meaning easier to see.

Key terms

growth/decay factor
the base b in a(b)t, the number you multiply by each time step to get the next value.
growth/decay rate
the percent change per step, equal to the factor minus 1, written as a decimal or percent.
initial value
the output when the input is 0, found by plugging in t = 0 or reading the standalone constant.
coefficient
the number multiplying a variable or power, controlling how fast that term changes the output.

Watch it explained

  1. How it worksHow do you tell which part of a times b to the x controls the starting size and which controls the speed?
  2. Worked exampleExplain what the 0.85 means in V(t) = 24000(0.85)t for a car that cost 24,000 dollars new.
  3. Worked exampleExplain what the -16 means in h(t) = -16t2 + 48t for a ball thrown straight upward.
  4. Going furtherWhat changes about the meaning of the constants when an exponential is rewritten with base 2?
  5. Common mistakeWhy isn't a growth factor of 1.07 the same thing as a growth rate of 1.07?

Check yourself

Your turneasy1 of 3
A car bought for 32410 dollars has value V(t) = 32410(0.862)t after t years, where 0.862 is less than 1. What does 0.862 represent?

End of topic.