Why can I multiply both sides by $dx$?
7474
What am I doing when I separate the variables of a differential equation?
25604
Why, historically, do we multiply matrices as we do?
101521
Both sides differentiation expression with integral
148632
How to expand $\cos nx$ with $\cos x$?
83610
Why does substitution work in antiderivatives?
1906
Multiplying by a $1\times 1$ matrix?
123220
Quick doubt about multiplying vectors by scalars
68310
Why can partial derivatives be exchanged?
34913
Why does basic algebra provide one value for $x$ when there should be two?
54294
Can the product $AB$ be computed using only $+, -,$ and reciprocal operators?
52946
If $X$ is an orthogonal matrix, why does $X^TX = I$?
61585
Why is area of a surface of revolution integral $2\pi y~ds$? not '$dx$'?
119763
Why separation of variables works in PDEs?
43385
Why is $\frac{\operatorname dy'}{\operatorname dy}$ zero, since $y'$ depends on $y$?
55188
Second order ODE - why the extra X for the solution?
153158
how to get $dx\; dy=r\;dr\;d\theta$
12726
Differentiating both sides of a non-differential equation