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  2. Proofs of trigonometric identities - Wikipedia

    en.wikipedia.org/wiki/Proofs_of_trigonometric...

    Two angles whose sum is π/2 radians (90 degrees) are complementary. In the diagram, the angles at vertices A and B are complementary, so we can exchange a and b, and change θ to π/2 − θ, obtaining: ⁡ (/) = ⁡

  3. AA postulate - Wikipedia

    en.wikipedia.org/wiki/AA_postulate

    We can then look at the sides around the parallels as transversals, and therefore the corresponding angles are congruent. Using this reasoning we can tell that similar triangles have congruent angles. Now, because this article is practically over, you might want to know what AA postulate can be used for. It is used proving the Angle Bisector ...

  4. Exact trigonometric values - Wikipedia

    en.wikipedia.org/wiki/Exact_trigonometric_values

    The values of sine and cosine of 30 and 60 degrees are derived by analysis of the equilateral triangle. In an equilateral triangle, the 3 angles are equal and sum to 180°, therefore each corner angle is 60°. Bisecting one corner, the special right triangle with angles 30-60-90 is obtained.

  5. Mathematical proof - Wikipedia

    en.wikipedia.org/wiki/Mathematical_proof

    For example, direct proof can be used to prove that the sum of two even integers is always even: Consider two even integers x and y. Since they are even, they can be written as x = 2a and y = 2b, respectively, for some integers a and b. Then the sum is x + y = 2a + 2b = 2(a+b). Therefore x+y has 2 as a factor and, by definition, is even. Hence ...

  6. Law of tangents - Wikipedia

    en.wikipedia.org/wiki/Law_of_tangents

    The law of tangents can be used to compute the angles of a triangle in which two sides a and b and the enclosed angle γ are given. From tan ⁡ 1 2 ( α − β ) = a − b a + b tan ⁡ 1 2 ( α + β ) = a − b a + b cot ⁡ 1 2 γ {\displaystyle \tan {\tfrac {1}{2}}(\alpha -\beta )={\frac {a-b}{a+b}}\tan {\tfrac {1}{2}}(\alpha +\beta ...

  7. Direct proof - Wikipedia

    en.wikipedia.org/wiki/Direct_proof

    In order to directly prove a conditional statement of the form "If p, then q", it suffices to consider the situations in which the statement p is true. Logical deduction is employed to reason from assumptions to conclusion. The type of logic employed is almost invariably first-order logic, employing the quantifiers for all and there exists.

  8. Theorem - Wikipedia

    en.wikipedia.org/wiki/Theorem

    The Pythagorean theorem has at least 370 known proofs. [1]In mathematics and formal logic, a theorem is a statement that has been proven, or can be proven. [a] [2] [3] The proof of a theorem is a logical argument that uses the inference rules of a deductive system to establish that the theorem is a logical consequence of the axioms and previously proved theorems.

  9. Spherical law of cosines - Wikipedia

    en.wikipedia.org/wiki/Spherical_law_of_cosines

    We define the quaternion u = (0, u) = 0 + u x i + u y j + u z k. The quaternion u is used to represent a rotation by 180° around the axis indicated by the vector u. We note that using −u as the axis of rotation gives the same result, and that the rotation is its own inverse. We also define v = (0, v) and w = (0, w).