Coalternate interior angles are equal
Two parallels crossed by a transversal, interior angles marked equal
A single geometric figure shows two parallel lines crossed by a diagonal, with the interior angles marked equal by dots. The accompanying note states the principle that when the interior angle equals the interior angle, the coalternate angles are equal. The sheet continues the study of parallels from the preceding pages of Notebook M.
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Equality of coalternate angles restated
Two parallels are cut by a transversal, and the interior angles are marked equal with dots. The note asserts that, the interior angle being equal to the interior angle, the coalternate angles are equal. This restates the parallel-line angle relation under examination.