The sine of
an angle θ (sin θ as it is usually written) can be defined using
a right-angled triangle, in which θ is one of the angles as shown,
by

sin θ = BC = Length of side opposite to θ

AC length of hypotenuse.

For example, if a ladder of length 5 metres leans against a wall
at an angle of 65^{o}

to the horizontal, the height h metres at which it touches the wall
is given by h/5 = sin 65^{o}

so h = 5 sin 65^{o} = 5 x 0.906 307 8

= 4.531 538 9

(to calculator accuracy)