Gilbert Strang, Introduction to Linear Algebra, 6th Edition (2023) 1. When can lines of lengths r,s,t form a triangle? They must satisfy the strict triangle inequalities r < s+t s < r +t t < r +s If we allow equality, the triangle will have angles of 0,0 and 180 degrees. If r > s+t, then one segment r is too long to complete a triangle. 2.