Mark13426
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 Jul 27 awarded Popular Question Mar 9 awarded Popular Question May 1 comment Proving that if $x$ is not free in $A$, then $(\exists x)(A\to B)\leftrightarrow(A\to(\exists x)(B))$ In axioms 4 and 5, x does not occur free. Sorry, I forgot to mention that. May 1 revised Proving that if $x$ is not free in $A$, then $(\exists x)(A\to B)\leftrightarrow(A\to(\exists x)(B))$ added 51 characters in body; edited title May 1 asked Proving that if $x$ is not free in $A$, then $(\exists x)(A\to B)\leftrightarrow(A\to(\exists x)(B))$ Apr 12 revised show that the following is logically valid edited title Apr 12 asked show that the following is logically valid Feb 20 awarded Student Feb 20 comment Derive: $(B\to A)\to(\neg A\to\neg B)$ @Gigili I should have added that translating a proposition to its definition is allowed, which I did according to hassan's answer. At one point in my proof, I had to use the first and second axioms as well as modus ponens. Feb 20 awarded Scholar Feb 20 comment Derive: $(B\to A)\to(\neg A\to\neg B)$ Thank you. This helped me solve the problem in less than a minute. Feb 20 accepted Derive: $(B\to A)\to(\neg A\to\neg B)$ Feb 20 awarded Editor Feb 20 revised Derive: $(B\to A)\to(\neg A\to\neg B)$ added 221 characters in body Feb 20 asked Derive: $(B\to A)\to(\neg A\to\neg B)$ Oct 12 comment Given an angle in radians, how could I calculate a 4x4 rotation matrix about the x, y, z axes? I need "the rotation matrix (a 4x4 matrix) about the x,y,z axes respectively by the specified angle in radians" Does that make sense? :) Oct 12 comment Given an angle in radians, how could I calculate a 4x4 rotation matrix about the x, y, z axes? I am not sure about what I need to do yet, but I think "Perhaps you mean a matrix for an affine transformation, which includes both rotations and translations? That can be written using a 4×4 matrix." is what I need. The matrix at the end of the calculation(s) must be 4x4. Oct 12 asked Given an angle in radians, how could I calculate a 4x4 rotation matrix about the x, y, z axes? Sep 22 asked Calculate surface normal of each equilateral triangle in a tetrahedron