Wikipedia:Reference desk/Archives/Mathematics/2015 January 23

= January 23 =

Polyhedra
What do you call a bipyramid with shaved base? In other words, make two identical Egyptian pyramids. Make cuts to create identical vertical walls. The structures' footprints/bases are now identical smaller squares. Glue the squares together so it's a 12-sided solid.

What's the Washington Monument's geometric shape? Is it different if the visible ground touching faces were vertical? Sagittarian Milky Way (talk) 06:32, 23 January 2015 (UTC)


 * Have any pics of the objects you have in mind ? StuRat (talk) 06:37, 23 January 2015 (UTC)


 * I've made it clearer. Sagittarian Milky Way (talk) 16:33, 23 January 2015 (UTC)


 * With regular faces, I think you're describing the elongated square bipyramid. If the Washington Monument had regular faces, it would be an elongated square pyramid. Its actual shape is sometimes called an obelisk, though MathWorld seems to disagree about the meaning of that term. (It's definitely an obelisk in the architectural sense.) -- BenRG (talk) 22:35, 23 January 2015 (UTC)

How do you solve this equation? (find all possible values of x)
$$18(-3^\frac{x}{2})^x - (-3^\frac{x}{2})^{2x} = 81$$ Thanks. The Transcendent One 17:10, 23 January 2015 (UTC)

Step 1. Simplify the equation

18*(((-3)^(x/2))^x)-(((-3)^(x/2))^(2*x)) == 81

18 * (-3)^((x^2)/2) - ((-3)^(x^2)) == 81

Step 2. Substitute x^2 with B

18 (-3)^(B/2) - (-3)^B == 81

step 3. Substitute (-3)^B with y

18 Sqrt[y] - y == 81

step 4. solve for all possible value of y (remembering that square root of y has two possible solutions

Step 5. solve for B

step 6. solve for x

QED

oh yeah! I almost forgot, you got to know if you want to work in the REAL domain or in the COMPLEX domain. In other words, do you want only the results in real numbers or in complex numbers. 172.56.32.205 (talk) 17:58, 23 January 2015 (UTC)


 * The foregoing answer reads $$-3^\frac{x}{2}$$ as meaning $$(-3)^\frac{x}{2}$$; it's a different problem if it means $$-(3^\frac{x}{2})$$. —Tamfang (talk) 08:05, 25 January 2015 (UTC)


 * Yes, I believe exponentation has a higher precedence than negation. Also, finding all possible solutions (which can be complex numbers in general) would be preferable. — Preceding unsigned comment added by The Transcendent One (talk • contribs) 15:17, 25 January 2015 (UTC)

A Maths Problem
Whats 9+10 and 6x6? — Preceding unsigned comment added by 78.133.9.40 (talk) 19:34, 23 January 2015 (UTC)


 * Since I can't imagine that you are simply asking for 9+10=19 and 6x6=36, which your could discover via Google, Wolfram Alpha, a calculator, or doing it in your head, I figure it must be a trick question and your "and" refers to a binary AND. Since 19 = 16 + 2 + 1 and 36 = 32 + 4, the answer to your question is zero. -- ToE 23:06, 23 January 2015 (UTC)


 * OTOH both 19 and 36 are non-zero which may be treated as TRUE; their conjunction then is TRUE, too. --CiaPan (talk) 14:57, 26 January 2015 (UTC)