Assumptions & Estimation

Geometry

Assumptions & Estimation

Lesson by Mike McGarry

Magoosh Expert

Lesson by Mike McGarry

Magoosh Expert

Summary

Visual estimation on ACT geometry questions helps narrow answer choices and check your work, even though it rarely solves a problem completely on its own. The most effective approach is to use diagrams strategically, compare lengths and angles, and keep track of reasonable size ranges for segments and angles.

Use visual estimation as a support tool rather than a full solution method; it works best for eliminating unlikely answers and confirming calculations.
On the paper-based ACT, simple physical references can help: the corner of the answer sheet can check whether an angle is close to 90 degrees, and a lightly marked edge can compare segment lengths.
In a diagram with a known side length of 6, a visibly longer segment can rule out answer choices smaller than that value, and comparison to a side labeled 14 can rule out choices that are clearly too large.
Even rough measurement can be enough to eliminate multiple options quickly, which is especially useful when time is short or the exact solution method is unclear.
In a triangle where sides 6 and 8 are shown, visual inspection can reveal that a third side is smaller than 6, immediately eliminating choices like 10 and 14.
Estimation also catches calculation mistakes: even if a 6-8-10 relationship comes to mind, the diagram shows that the side in question is not the hypotenuse and cannot be the largest side.
Approach every ACT diagram by judging reasonable ranges for line segments and angles so that the picture becomes a tool for both elimination and verification.

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