Graphing Functions

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PSAT Math › Graphing Functions

Questions 1 - 4
1

A coordinate plane shows an exponential curve labeled $f(x)$. It passes through $(0,2)$ and $(1,4)$ and approaches $y=0$ as $x$ decreases. Which equation matches the graphed function? Be careful: confusing the base or the initial value would produce a similar-looking curve.

$y=4^x$

$y=2^x$

$y=2\cdot 4^x$

$y=2\cdot 2^x$

Explanation

The question seeks the equation of an exponential function passing through (0, 2) and (1, 4), approaching y = 0 as x decreases. The graph illustrates a curve starting near the x-axis on the left, rising through (0, 2) and (1, 4), characteristic of exponential growth with a horizontal asymptote at y = 0. To match this, use the general form y = a * $b^x$; the point (0, 2) gives a = 2, and (1, 4) gives 2 * b = 4, so b = 2, resulting in y = 2 * $2^x$, which aligns with the visual growth rate. This connects the initial value and doubling factor to the plotted points algebraically. A common error is selecting y = $2^x$, which starts at (0, 1) instead of (0, 2), or y = $4^x$, which also starts at 1 but grows faster. Mistaking the base for 4 while keeping the multiplier can lead to y = 2 * $4^x$, which passes through (0, 2) but reaches (1, 8), not (1, 4). For such questions, plug in the given points to verify the equation rather than relying solely on the curve's shape.

2

The coordinate plane shows a line segment graphed for a function $f$. The segment starts at a closed dot at $(-4,3)$ and ends at an open dot at $(2,-3)$. What is the domain of the graphed function? Read the endpoint symbols carefully to avoid including an excluded value.

$[-4,2)$

$[-4,2]$

$(-4,2]$

$(-4,2)$

Explanation

This question asks for the domain of a function represented by a line segment from a closed dot at (-4, 3) to an open dot at (2, -3). The graph shows a straight line connecting these points, with the closed dot indicating inclusion of x = -4 and the open dot excluding x = 2. The domain, or set of x-values, thus ranges from -4 to 2, inclusive at -4 and exclusive at 2, expressed in interval notation as [-4, 2). This connects the visual endpoint symbols to the algebraic inclusion or exclusion in the domain. A key error is misreading the dots, such as including 2 for a closed interval like [-4, 2] or excluding -4 for (-4, 2). Another mistake involves confusing domain with range, focusing on y-values instead. Always check endpoint symbols carefully and use test points near the ends to confirm inclusion when determining domains from graphs.

3

The coordinate plane shows a line that crosses the $y$-axis at $-1$ and passes through the point $(4,3)$. Which equation matches the graphed line? Several choices have a similar slope, so use both the intercept and the plotted point.

$y=\tfrac{3}{4}x-1$

$y=x-1$

$y=x+1$

$y=\tfrac{1}{2}x-1$

Explanation

This question requires the equation of a line crossing the y-axis at -1 and passing through (4, 3). The graph depicts a straight line with y-intercept at (0, -1) and another point at (4, 3), allowing calculation of the slope. The slope m = (3 - (-1))/(4 - 0) = 4/4 = 1, leading to y = x - 1 in slope-intercept form, which matches both points algebraically and visually. This connects the intercept and slope to the line's position and steepness on the graph. A key error is miscalculating the slope, such as to 3/4 or 1/2, resulting in lines that miss (4, 3). Another mistake is using y = x + 1, which has the wrong intercept. Always use both the intercept and a given point to verify the slope when multiple options have similar forms.

4

The coordinate plane shows a decreasing exponential function with horizontal asymptote $y=0$. The curve passes through $(0,8)$ and $(1,4)$. What is the value of the function at $x=2$? Use the plotted points to determine the pattern rather than guessing from the curve’s shape.

$4$

$16$

$1$

$2$

Explanation

This question requires finding the function value at x = 2 for a decreasing exponential with asymptote y = 0, passing through (0, 8) and (1, 4). The graph shows a curve approaching y = 0 from above as x increases, starting higher on the left and decreasing through the given points, indicative of exponential decay. Using y = a * $b^x$, (0, 8) gives a = 8, and (1, 4) gives 8b = 4 so b = 1/2; at x = 2, y = 8 * $(1/2)^2$ = 2, connecting the decay factor to the pattern in points. This algebraic model matches the visual halving each unit increase in x. A key error is assuming a base like 2 or 4 without the decay, leading to growth instead of decrease. Guessing from shape without calculation might suggest 1, 4, or 16, missing the pattern. Use consecutive points to find the common ratio and extend it to the desired x-value for accuracy.