Solve for x: 2 over 3 (x − 4) = 2x
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I want to talk about X's. I'll start by talking about this X - a cross (photo 1). Specifically, a cross made of two perpendicular lines - lines that meet each other at right angles all around. What's special about a cross? There's the religious significance, but in our case, there's also the <em>symmetry</em>. The quick, intuitive definition of symmetry is some quality that a thing has where, if you do something to it, it'll still look the same. Your body is symmetric; the left side of our bodies looks like we took the right side and flipped it - they're mirror images of each other.
One of the symmetries of this cross is right here. (Picture 2) It splits the cross into <em>two identical halves</em>, one a mirror image of the other. Most importantly, <em>the angles on each side of the line are identical</em>. We can draw this same mirror into our problem, too (picture 3). This symmetry tells us that those two unknown angles are exactly the same - <em>equal</em>. So, if angle MNJ is 5x+2 and angle LNK is 3(x+14), we now know that
5x + 2 = 3(x + 14), or, getting our algebra sorted:
5x + 2 = 3x + 42
2x = 40
x = 20
Now that we know x, we can find MNJ - which, because X's are symmetric, is exactly the same angle as LNK. Crunching the numbers, we find
m∠MNJ = m∠LNK = 5(20) + 2 = 100 + 2 = 102°.
Side note: the technical term for pairs of angles like MNJ and LNK is vertical angles. What we've shown here is, because of the symmetry of intersecting lines, vertical angles are always equal.
