A 120-inch strip of metal 12 inches wide is to be made into a small open trough by bending up two sides on the long side, at rig
ht angles to the base. The sides will be the same height, x. If the trough is to have a maximum volume, how many inches should be turned up on each side?
Looks like the larger triangle is equilateral (well, it must be, otherwise finding the area is impossible). That means the inradius is 6 and the outradius is 12, which is to say the altitudes (equal to the height, in this case) of this triangle have length 6 + 12 = 18.
In the smaller right triangle, the length of the missing leg is half the base of the larger triangle; call this length By the Pythagorean theorem,