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For what value of [tex] x [/tex] is [tex] \cos(x) = \sin(14^\circ) [/tex], where [tex] 0^\circ \ \textless \ x \ \textless \ 90^\circ [/tex]?

A. [tex] 14^\circ [/tex]
B. [tex] 31^\circ [/tex]
C. [tex] 28^\circ [/tex]
D. [tex] 76^\circ [/tex]


Sagot :

To find the value of [tex]\( x \)[/tex] for which [tex]\( \cos(x) = \sin(14^\circ) \)[/tex] in the interval [tex]\( 0^\circ < x < 90^\circ \)[/tex], we can use the co-function identity for trigonometric functions:

[tex]\[ \cos(x) = \sin(90^\circ - x) \][/tex]

Given the equation [tex]\( \cos(x) = \sin(14^\circ) \)[/tex], we can equate the expressions:

[tex]\[ \cos(x) = \sin(14^\circ) \][/tex]

By the co-function identity, we know:

[tex]\[ \cos(x) = \sin(90^\circ - x) \][/tex]

Therefore, we can write:

[tex]\[ \sin(90^\circ - x) = \sin(14^\circ) \][/tex]

Since the sine function is positive and strictly increasing in the range [tex]\( 0^\circ \)[/tex] to [tex]\( 90^\circ \)[/tex], the equality [tex]\( \sin(90^\circ - x) = \sin(14^\circ) \)[/tex] implies:

[tex]\[ 90^\circ - x = 14^\circ \][/tex]

Solving for [tex]\( x \)[/tex], we get:

[tex]\[ 90^\circ - x = 14^\circ \][/tex]

[tex]\[ x = 90^\circ - 14^\circ \][/tex]

[tex]\[ x = 76^\circ \][/tex]

So, the value of [tex]\( x \)[/tex] is [tex]\( 76^\circ \)[/tex].

Therefore, the correct answer is:
[tex]\[ \boxed{76^\circ} \][/tex]