Answer:
cosine
Step-by-step explanation:
"cos" is an abbreviation for "cosine," the name of the trig function that relates the hypotenuse of a right triangle to the length of the side adjacent to the angle of interest.
For an angle measured in radians, the value of the cosine of the angle is defined by the power series shown in the second attachment. Any scientific or graphing calculator can evaluate this function. Generally, best accuracy from a calculator is obtained for angles between -2π and +2π radians, or ±360°. Calculators generally can use angles expressed in degrees or radians, so care is required to make sure the calculator mode matches the units you are using. (Spreadsheet functions typically require angles to be in radians.)
The cosine function is periodic, with a period of 2π radians. Its value for x=0 is 1. A graph of the cosine and sine (abbreviated "sin") functions is shown in the third attachment. Each is a horizontal shift of the other.
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The three primary trig functions relate pairs of sides of a right triangle to one of the acute angles. The relationship between trig functions and sides is summarized in the mnemonic SOH CAH TOA. The first attachment shows what that means.
