sin@/2 sin 7@/2  +sin 3@/2 sin 11@/2=sin2@ sin5@

let @/2 = t

now, sin@/2 sin 7@/2  +sin 3@/2 sin 11@/2 =sint sin7t + sin3t sin11t

=1/2[(cos6t - cos8t )+(cos8t - cos14t)]

=1/2[ cos6t - cos14t ]

=1/2[ -2 sin10t (-sin4t )]

=sin10t sin4t = sin10@/2  sin4@/2 = sin5@ sin2@

hence proved