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Definition of cosine
1 : a trigonometric function that for an acute angle is the ratio between the leg adjacent to the angle when it is considered part of a right triangle and the hypotenuse
2 : a trigonometric function cos θ that for all real numbers θ is given by the sum of the alternating series and that is exactly equal to the cosine of an angle of measure θ in radians : a trigonometric function cos θ that for all real numbers θ is given by the sum of a series of which the first term is 1 and succeeding terms alternate in sign with the kth term consisting of a fraction whose numerator is θ raised to the exponent 2(k  1) and whose denominator is the factorial of 2(k  1) and that is exactly equal to the cosine of an angle of measure θ in radians
Recent Examples of cosine from the Web

The link was so clear it could be described in derivative sines and cosines, mathematical concepts taught in college trigonometry.
These example sentences are selected automatically from various online news sources to reflect current usage of the word 'cosine'. Views expressed in the examples do not represent the opinion of MerriamWebster or its editors. Send us feedback.
Origin and Etymology of cosine
New Latin cosinus, from co + Medieval Latin sinus sine
COSINE Defined for English Language Learners
cosine
playDefinition of cosine for English Language Learners

geometry : the ratio between the long side (called the hypotenuse) and the side that is next to an acute angle in a right triangle
Learn More about cosine

Britannica English: Translation of cosine for Arabic speakers Britannica.com: Encyclopedia article about cosine
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