/sci/ - Science & Math

/sci/, please help me with this review question.
I don't want an answer, just the steps to solve it (what I should do).

(1-tan^2(x)) / (1+tan^2(x)) = cos2x

ITT: somebody explains thermochemistry to me as quickly and concisely as possible because I only just started to study right now for my test tomorrow morning

Why can't the heat of reaction for the combustion of magnesium be measured directly? Is it because the reaction is exothermic?

When dissolving a covalent substance in water, what 3 types of bonds could be formed?

Is it London force bonds/dipole-dipole bonds/hydrogen bonds? Correct me if I'm wrong.

Question from a friend:
"hey, do you know what cyclohexene tests for?"

I don't. I'm not an orgo chem student.
Does anyone have an answer?

Where can I purchase basic lab chemicals (ionic solutions, pure metals, acids, gases, etc) without being asked if I'm a teacher or a terrorist? I just want to do some chemistry without having to sign up for a course and doing any bullshit homework.

accurate?

I'm thinking of becoming an engineer