7 is incorrect. The answer should be -4. Here's how I'd derive it:

With
, we should expect
. If
, then

Also,

so we should expect
and

You seem to be taking

but this is not an identity.
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8 is incorrect. Just a silly mistake, you swapped the order of the terms in the numerator. It should be

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9. Use the identity from (7).
lies in the second quadrant, so

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12.




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13.




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14.



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15.



Answer:
-71, and also 71
Step-by-step explanation:
It's Simple Subtraction or if your doing 85-14 its gonna be 71
Answer:
It is a whole number
Step-by-step explanation:
It has not decimals or fractions
Answer:
ab + bc + ca = 40
Step-by-step explanation:
Using the expansion identity
(a + b + c)² = a² + b² + c² + 2(ab + bc + ca) , that is
9² = 1 + 2(ab + bc + ca)
81 = 1 + 2(ab + bc + ca) ← subtract 1 from both sides
80 = 2(ab + bc + ca) ← divide both sides by 2
40 = ab + bc + ca