๐ Quick Analysis Using Graphs
Given: f(x)=kx+b
โฅ Determine the sign of the slope by observing the quadrants the line passes through
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Positive slope: k>0: The line goes from bottom-left to top-right, passing through the first and third quadrants
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Negative slope: k<0: The line goes from top-left to bottom-right, passing through the second and fourth quadrants
โฅ Determine the sign of the y-intercept( b ) by observing where the line crosses the y-axis
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Positive y-intercept: b>0: The intersection point is above the x-axis
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Negative y-intercept: b<0: The intersection point is below the x-axis
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No y-intercept: b=0: The intersection point passes through the origin (0,0)
๐ก This method helps you quickly eliminate incorrect answer choices. Sometimes you don't even need to perform actual calculations - you can select the correct answer just by analyzing the graph.
Example:

Explanation:
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Based on the graph, we can first observe that the function passes through the second and fourth quadrants, which means the slope is negative. At this point, we can immediately eliminate options A and D (both have a slope of 1/3). Next, we only need to choose between B and C.
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Then, looking at where the line intersects the y-axis, we see it's below the x-axis-that is, on the negative half of the y-axis. This means the y-intercept is negative, so option B is also eliminated (its y-intercept is +6).
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Thus, the correct answer is C.
In this process, we didn't pick points on the graph and calculate the function's equation to solve it. We could determine the answer just by "looking at the graph directly"!
๐ Quick Calculation of Slope
- Master the slope formula for two points (x1โ,y1โ) and (x2โ,y2โ):
k=x2โโx1โy2โโy1โโ
- Do not swap the numerator and denominator.
- If the SAT problem provides two intercept points (e.g., (3,0) and (0,4)), the slope is:
k=โx-intercepty-interceptโ=โ34โ.
๐ Intercept Shortcut Method
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Memorize these conclusions to save time during exams:
- x-intercept =โkbโ (derived by setting y=0 in 0=kx+b)
- y-intercept =b (obtained directly by setting x=0)
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Note: If the equation is in standard form Ax+By=C, then:
- x-intercept =ACโ
- y-intercept =BCโ (no rearrangement needed).
๐ Two-Point Form to Slope-Intercept Form
- Given two points, first calculate the slope k, then use the point-slope form:
yโy1โ=k(xโx1โ)
Finally, rearrange it to:
y=kx+b.
๐ Quick Graph Reading
- y-intercept: Observe the y-coordinate where the line crosses the y-axis.
- Slope: Count the grid units for runriseโ (e.g., a point moves right 2 units and up 1 unit to return to the line โ k=21โ).
๐ Keyword Translation in Word Problems
- Terms like "Initial value", "base fee", "fixed cost", etc. โ y-intercept b.
- Terms like "Rate of change", ".. per ..", etc. โ Slope k.
๐ Extreme Value Verification
- Test x=0 or x=1 to validate the equation's reasonability.
๐ Reverse Substitution for Multiple-Choice
- For complex functions (e.g., fractions), if the question asks for f(3), substitute the given options backward to check which fits the function.
๐ Simplify Equations
- First, check if the function can be simplified (e.g., 2g(x)=4f(x)โ8 simplifies to g(x)=2f(x)โ4). This makes analysis clearer and reduces calculation difficulty.