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MENTAL MATH: TiPs & TrIcKs for...

Trigonometry: Sine

Use this to find out the sine of angles of 30°, 45° and 60° degrees. [Opposite leg/hypotenuse]. Sin(30) = 1/2

(fig 9)

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

(fig 10)

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

The sine of any angle in the II quadrant, as in an angle from 90° to 180°, is positive. (Both the tangent and the cosine are negative.)

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

The sine of any angle in the IV quadrant, as in an angle from 270° to 360°, is negative (as well as the tangent). Only the cosine is positive.