Question

Which relationship in the triangle must be true?

A
c
b
C
B
а
sin(B) = sin(A)
sin(B) = cos(90 - B)
COS(B) = sin(180 - B)
cos(B) = (A)

Answer 1

sin(B) = cos(90 - B).

Explanation:

To answer this question, you must understand SOH CAH TOA.

SOH = Sine; Opposite divided by Hypotenuse

CAH = Cosine; Adjacent divided by Hypotenuse

TOA = Tangent; Opposite divided by Adjacent

I roughly drew a triangle for reference. Let's say we have a 3-4-5 triangle.

As you can see, sin(b) does not equal sin(a). To get the sine of an angle, you would do opposite over hypotenuse. For angle B, that would be 3/5, while for angle A, that would be 4/5.

As stated above, sin(B) is 3/5. Now, if you did cos(90 - B), it would be the same thing as cos(A). This is because the triangle is a right triangle. Since a triangle has 180 degrees, and one angle is a right triangle, the other two angles will add up to be 90 degrees. So, 90 - B = A. cos(A) is the same thing as adjacent over hypotenuse, which is 3/5. So, sin(B) = cos(90 - B) must be true.

Let's just check the others to make sure they are false.

cos(B) = 4/5.

sin(180 - B) is basically the same thing as sin(A + C), which is definitely NOT 4/5.

cos(B) = 4/5, which is NOT the same as A.

So, your answer is sin(B) = cos(90 - B).

Hope this helps!