Hello!
In geometry, the Triangle Inequality theorem states that the sum of any two sides of the triangle must be greater than the third side, the one not added to another. This must work for all 3 combos of pairs of sides.
So we can look at your first problem, two sides are 18 and 11. The first thing you want to see is how much would have to be added to 11 so it would be greater than 18, as 11 + x > 18 is one of the pairs. If we subtract 11 from both sides of that inequality, 11 + x - 11 > 18 - 11, then you get x > 7. And since there's only one selection where it has 7 < x, or 7 < x < 29, then you know your answer is that.
Next question. We can go through the answer choices. 4 would obviously not work, as 8 + 4 = 12, and that's less than 15. 7 wouldn't either, as 8 + 7 = 15, and 15 = 15, and it has to be greater. Same case for 23, as 15 + 8 = 23, and that wouldn't work. So the only choice that ends up being correct is 10. You can check that by doing, 15 + 8 = 23, 23 > 10. 15 + 10 = 25, 25 > 8. 8 + 10 = 18, 18 > 15.
