# Tagged Questions

Questions related to memory devices that help learners recall larger pieces of information, especially in the form of lists like characteristics, steps, stages, parts.

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### How can I remember whether finite or countable cartesian product of countable set is countable

I always forget this result Is cartesian product of countable set countable under finite or countable cartesian products? Is there a good way to remember this? Like a proof sketch where the ...
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### Remembering multiplication of these two numbers: $7 \times 8 = 56$ and $9 \times 6 = 54$

I have almost mastered multiplication table up to 9x9 however, I'm having problems with the following two. 7 x 8 = 56 and 9 x 6 = 54 For some reason my brain thinks that 56 and 54 are somewhat the ...
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### Easy way of memorizing values of sine, cosine, and tangent

My math professor recently told us that she wanted us to be able to answer $\sin\left(\frac{\pi }{2}\right)$ in our head on the snap. I know I can simply memorize the table for the test by this Friday,...
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### Remembering that $\sin^2(\theta) = 1/2 - 1/2\cos(2\theta)$?

How do you remember this for integrals? It doesn't seem obvious and I can never remember it when I come across it in integrals.
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### Injection vs. Surjection: Mnemonic to remember which is which

What are some mnemonics to help one remember that Injection = One-to-one and Surjection = Onto? The only thing I can think of is 1njection = 1-1.
Specific question: how can I improve on mentally performing calculations like these : $$(78-59)\times23-8\%\times(270+130)$$ [2\text{digit}+-2\text{digit}]\times2\text{digit}+-x\%\text{of}[3\text{...