If #sin^(-1)(x)=theta#
then
#color(white)("XXX")# based on a unit circle (hypotenuse #= 1#)
#color(white)("XXX")# the side opposite #theta# will have a length of #x#
#color(white)("XXX")#and
#color(white)("XXX")#the adjacent side will have a length of #sqrt(1-x^2)# (based on Pythagorean Theorem)
#tan(sin^(-1)(x)) = tan(theta) = ("opposite")/("adjacent") = x/(sqrt(1-x^2))#