how do you cross multiply in trigonometry?

Answer 1

cross multiply is indepent from trigonometry, it's applied to equations in general.

for instance if you have this equality:

2 /3 x = 5/7 y

you could multiply the whole expression by 3, and by 7 so that you don't have denominators anymore => that's cross multiplication

3*7*(2/3) x = 3*7*(5/7) y

(3/3) * 7 * 2 x = 3*5 * 7 / 7 y

7 * 2 x = 3 * 5 y

14 x = 15 y

Answer 2

cross-multiply *entire* sides (the left by the right and the right by the left)

The only pitfall in cross-multiplying with denominators is that you have to check not to multiply by zero, which otherwise would need a special sub-treatise.