Question

If the sin 90º = 1, then which statement is true?

Answer 1

in a right-triangle, like in the 1st Quadrant, cosine and sine are complements, so then

sin(x) = cos(90° - x)

cos(x) = sin(90° - x)

for this case, sin(90°)  = cos(90° - 90°), or just cos(0°) = 1.