BrownMath.com → Trigonometry → 10-Minute Trig
Updated 27 Oct 2020

# 10-Minute Trig

Summary: Use this page to jog your memory with the basic facts of right-angle trigonometry. Please see Trig without Tears for explanations of these quick notes and many more topics.

circumference of circle: 2πr
angle around circle (like clock hand): 2π radians or 360°
Therefore 2π = 360°, or π = 180°, or 1 radian = 180°/π.

Press `[MODE]` to check calculator mode (radian or degree).

## B. Trig Ratios of Acute Angles  SOHCAHTOA sin A = cos(π/2−A) cos A = sin(π/2−A) tan A = cot(π/2−A) cot A = tan(π/2−A) sec A = csc(π/2−A) csc A = sec(π/2−A) cot A = 1 / tan A sec A = 1 / cos A csc A = 1 / sin A

## C. Trig Functions of Any Angle

The definitions based on an acute angle in a right triangle extend to trig functions of any angle: sin θ = y/r cos θ = x/r tan θ = sin θ / cos θ = y/x cot θ = 1 / tan θ sec θ = 1 / cos θ csc θ = 1 / sin θ Pythagorean theorem ( y² + x² = r² ) leads to sin² θ + cos² θ = 1> r is always >0, so signs of functions in any quadrant pop right out from signs of x and y in that quadrant.  Do quadrant angles by reference to x y r, e.g. cos 0° = 1/1 = 1 and sin π = 0/−1 = 0. Use reference angle (acute angle between terminal side and x axis) to relate function values to values for an acute angle.

## D. Trig Functions of Special Angles  c = 1 (given), a = b By Pythagoras, a = b = √2 / 2 c = 1 (given), b = c/2 = ½ By Pythagoras, a = √3 / 2
0 = 0° π/6 = 30° π/4 = 45° π/3 = 60° π/2 = 90°
sin θ 0 1/2 2 / 2 3 / 2 1
cos θ 1 3 / 2 2 / 2 1/2 0
tan θ 0 3 / 3 1 3 undef.

## E. Inverse Functions arcsin 0.65 or sin-10.65 means the angle whose sine is 0.65. That’s not the same as 1/sin 0.65
Function ranges:  −π/2 ≤ arcsin x ≤ +π/2,  0 ≤ arccos x ≤ +π,  −π/2 < arctan x < +π/2

Function composition (see diagram at right):
What is e.g. cos( arctan x )?
Solution: arctan x is the angle whose tangent is x; call it A. Then you must find cos A. Use Pythagoras to find the third side, √x²+1, then read off function value: cos A = 1 / √x²+1.

## What’s New

• 27 Oct 2020: Converted from HTML 4.01 to HTML5; cleaned up some formatting; italicized variable names.
• 17 Aug 2015: Moved from OakRoadSystems.com to BrownMath.com.
• (intervening changes suppressed)
• 10 July 2003: New article, based on a summary I prepared for my precalculus class in March 2002.