Mark all the expressions that are equal to sin A

We will see that the expressions equivalent to Sin(A) are:
Well, remember that for a triangle rectangle we have the rule:
Sin(θ) = (opposite cathetus)/(hypotenuse)
in the case of A, the opposite cathetus is BE, and the hypotenuse is AB, but that option is not there.
Notice that C is an angle equal to A (as the angle B does not changes and angles E and D are of 90°, so A must be equal to C).
In the case of C the hypotenuse is CB and the opposite cathetus is DB, then:
Sin(A) = DB/CB
We also have that:
Sin(A) = Sin(C) = Sin(∠DCB)
Finally, with the rule:
cos(θ) = (adjacent cathetus)/(hypotenuse)
We can get:
Cos(B) = DB/CB = Sin(A)
then:
Cos(B) = Sin(A).
So the correct options are:
If you want to learn more about trigonometry, you can read:
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