Inverse Trigonometric Functions Math Example 4

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Example 4

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Solve 2sinโก(x)โˆ’1=02\sin(x)-1=0 on [0,2ฯ€][0,2\pi] using arcsinโก\arcsin, and explain why there are two solutions.

Solution

  1. 1
    Isolate: sinโก(x)=12\sin(x)=\frac{1}{2}. Primary solution: x=arcsinโกโ€‰โฃ(12)=ฯ€6x=\arcsin\!\left(\frac{1}{2}\right)=\frac{\pi}{6}.
  2. 2
    Second solution: sine is also 12\frac{1}{2} in the second quadrant at x=ฯ€โˆ’ฯ€6=5ฯ€6x=\pi-\frac{\pi}{6}=\frac{5\pi}{6}.
  3. 3
    So x=ฯ€6x=\frac{\pi}{6} and x=5ฯ€6x=\frac{5\pi}{6}. The arcsinโก\arcsin function only gives the first; the second must be found using symmetry.

Answer

x=ฯ€6x = \dfrac{\pi}{6} and x=5ฯ€6x = \dfrac{5\pi}{6}
Inverse trig functions return only one value (the principal value) by design. Because sine is many-to-one, equations like sinโก(x)=c\sin(x)=c have multiple solutions in [0,2ฯ€][0,2\pi]. We must use the unit circle's symmetry to find all solutions.

About Inverse Trigonometric Functions

Functions that reverse the trigonometric functions: given a ratio, they return the corresponding angle. arcsinโก\arcsin, arccosโก\arccos, and arctanโก\arctan are the inverses of sinโก\sin, cosโก\cos, and tanโก\tan on restricted domains.

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