cos 75 + cos 15 / sin 75 sin 15

cos 75 + cos 15 / sin 75 sin 15
= 1/4√6 - 1/4√2 + 1/4√6 + 1/4√2 / [1/4√6 + 1/4√2][1/4√6 - 1/4√2]
= 2/4√6 / [3/8 - 1/2]
= 1/2√6 / [3/8 - 4/8]
= 1/2√6 / [-1/8]
= 1/2√6 /-1/8
= -4√6