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Domain, Range & Principal Value

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Estimated time: 9 minutes
CBSE: Class 12

Introduction

Inverse trigonometric functions are used to find the angle when the value of a trigonometric ratio is known. Since trigonometric functions are not one-one over their full domains, their domains are restricted so that inverse functions can be defined properly. This is why domainrange, and principal value are central ideas in this chapter.

CBSE: Class 12

Definition: Inverse trigonometric Function

The inverse trigonometric functions are the inverse forms of trigonometric functions after suitable domain restriction. They are written as:

  • \[\sin^{-1} x\]

  • \[\cos^{-1} x\]

  • \[\tan^{-1} x\]

  • \[\cot^{-1} x\]

  • \[\sec^{-1} x\]

  • \[\csc^{-1} x\]

Important:

  • \[\sin^{-1} x\]

does not mean 1/sin⁡x. It means the angle whose sine is x.

CBSE: Class 12

Definition: principal value

The value returned by an inverse trigonometric function is called its principal value. It is the unique angle chosen from the standard restricted interval for that function.

CBSE: Class 12

Standard domain and principal value ranges

Inverse Trigonometric Function Domain Range (Principal Value)
sin⁻¹ x [-1, 1] [-π/2, π/2]
cos⁻¹ x [-1, 1] [0, π]
cosec⁻¹ x R − (-1, 1) [-π/2, π/2] − {0}
sec⁻¹ x R − (-1, 1) [0, π] − {π/2}
tan⁻¹ x R (-π/2, π/2)
cot⁻¹ x R (0, π)
CBSE: Class 12

Key Points: Domain, Range & Principal Value

  • Inverse trigonometric functions give angles corresponding to known trigonometric values.

  • Their domains are restricted because ordinary trigonometric functions are not one-one on full domains.

  • Principal value means the standard angle selected from a fixed interval.

Maharashtra State Board: Class 12

Graph for Time Series

Trend line by semi-averages

Trend line by semi-averages

Magazine subscribers, 1976–1983. Step through the five stages to fit a trend line graphically and read off the rate of increase.

Trend line for magazine subscribers fitted by the method of semi-averages Stepwise construction. Eight yearly subscriber counts from 1976 to 1983 are plotted, divided into two halves of four years each, the mean of each half is plotted at the middle year of that half, and a straight line is drawn through the two semi-average points to give the trend line. 10 12 14 16 18 20 22 24 1976 1977 1978 1979 1980 1981 1982 1983 Year Subscribers (millions) First half Second half 12 11 19 17 19 18 20 23 14.75 20.00 Trend line

Step 1: Plot the data

Plot the 8 yearly subscriber counts on a Year vs Subscribers chart. The zigzag captures every year-to-year wobble — useful, but noisy. The publisher wants the underlying direction, not the noise.

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