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MENTAL MATH: TiPs & TrIcKs for...

Trigonometry: Cosine

Use this to find out the cosine of angles of 30°, 45° and 60° degrees. [Adjacent leg/hypothenuse]. Cos 45 = 1/√2

 

 
     
(fig 9)
 

 

This circle helps to figure out if the cosine of any given angle in the I, II, III or IV cuadrants is positive or negative.


(fig 10)

The cosine of any angle in the I cuadrant, as in an angle from 0° to 90°, is positive (as well as the tangent and sine).

The cosine of any angle in the II cuadrant, as in an angle from 90° to 180°, is negative (as well as the tangent). Only the sine is positive.

The cosine of any angle in the III cuadrant, as in an angle from 180° to 270°, is negative (as well as the sine). Only the Tangent is positive.

The cosine of any angle in the IV cuadrant, as in an angle from 270° to 360°, is positive. (Both the tangent and sine are negative.) 

 

     
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