Drew Christianson
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 Sep 23 awarded Nice Answer Sep 23 answered How is $e^x$ read aloud? Sep 22 comment calculus of variations question @Rut welcome to math.se! Posts of this nature should usually be comments on the main question rather than answers. The latter are generally reserved for complete solutions to the problem at hand. Sep 22 comment Sum of Infinite Surds I'm not quite sure I understand precisely the value you're computing. Is it: $$\sqrt{10+\sqrt{100+\sqrt{1000+\cdots}}}$$ Sep 21 revised Is the square root of -1 rational? added 35 characters in body Sep 21 comment Trig to determine distance: boat on course parallel to shore. Draw it out first. Can you see how the boat's path in the problem forms a triangle? Sep 21 answered Is the square root of -1 rational? Sep 21 accepted Intuition for the Product of Vector and Matrices: $x^TAx$ Sep 18 answered Cubic equation - finding coefficients Sep 18 comment Cubic equation - finding coefficients I agree with the above comments that this is not at all the appropriate form for this question. That said, if I interpret your variable names correctly, you fit the parabola with equation $$ax^2+bx+c$$ through three points, $p_1=(x_1,y_1), p_2=(x_2,y_2), p_3=(x_3,y_3)$ and propose to do so with the equations $$a=\frac{((y_2-y_1)(x_1-x_3))+((y_3-y_1)(x_2-x_2))}{(x_1-x_3)(x_2^2-x_1^2)+(x_2-x‌​_1)(x_3^2-x_1^2)}$$ $$b= \frac{(y_2-y_1)-a(x_2^2-x_1^2)}{x_2-x_1}$$ $$c = y_1-ax_1^2 - bx_1$$ and are asking for a similar equation to fit a cubic through three (or four?) points. Sep 18 comment Intuition for the Product of Vector and Matrices: $x^TAx$ @Alex I know what matrix multiplication means, I'm asking for ways to perform it quickly, or at least 'know' categorical/broad stroke information about the products at a glance. Sep 17 asked Intuition for the Product of Vector and Matrices: $x^TAx$ Sep 17 revised Is it possible to practice mental math too often? edited tags Sep 17 comment Is it possible to practice mental math too often? George, what sort of arithmetic are you practicing? Is your question more about getting basic times tables down, or how to do more complicated 2+ digit multiplication/mulitiplication by fractions? Sep 8 revised How do these cross products work Added vector hats Sep 8 comment How do these cross products work I know links aren't always the most helpful, but I would refer you to: en.wikipedia.org/wiki/Cross_product#Algebraic_properties I often find it helpful to have the list of properties as reference when learning how to deal with a new operation (like the cross product). Sep 8 suggested approved edit on How do these cross products work Sep 7 awarded Yearling Jun 19 revised $E[x\mid x>1]$ if $X \sim \exp(\lambda)$ Convert to Latex Jun 19 suggested approved edit on $E[x\mid x>1]$ if $X \sim \exp(\lambda)$