I got request to explain this answer........................ OK............ that's simple. You may know that sinΘ is always equal to hieght of the triangle / diagonal of triangle. Now, if u r confused about what is diagonal and what is hieght, then, in right angled triangle, assume this is trianlge ABC. http://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Rtriangle.svg/220px-Rtriangle.svg.png
; And <C=90°. And <A=Θ°. Now, height and base decieded on the basis of given angle, here <A=Θ° is given to you. Now, from <A, two sides arises, one is diagonal "c" (AB) because is opposite to <C. other side of triangle which is attached to <A is the base of triangle which is "b" (AC) . And the remaining "a"(BC) is the height of that trianlge, Now, sin Θ= height / diagonal = BC/AB = a/c. hope u'll get it.