cos(20) = sin(70)
note how 20+70 = 90
The general rule is that if x+y = 90, then cos(x) = sin(y). The rule works in reverse as well. Another way to phrase the rule is cos(x) = sin(90-x).
So,
Note: if the computer doesn't like the answer of "sin 70" then try "sin(70)" and see how that works out. It's also possible that the teacher wants the single numeric value of "70" in the first box rather than the "sine" part to go along with it. Though that would be strange and incorrect since cos(20) = 70 is a false statement.
Women can be seated in 89 ways.
Let Sn be the number of possible seating arrangements with n women. Consider n≥3 and focus on the rightmost woman. If she goes back to her seat, then there are Sn−1 ways to seat the remaining n−1 women. If he is sitting in the penultimate seat, then the woman who was sitting there before must now sit in the rightmost seat.
This gives us Sn−2 ways to seat another n−2 woman, so we get the recursion Sn=Sn−1+Sn−2. Starting with S1=1 and S2=2 we can calculate S10=89.
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The answer is C, because in the beginning there where 4 carbon, then when added the oxygen, there was 1 left
