x1 is directly/linearly proportional to x2 via coefficient/constant of proportionality x3 and background/constant offset x4; there exists constants x3 and x4 such that x1 = x3×x.
x2 may be an incomplete list of quantities. These should be variable. x3 should be a fixed constant for the universe, situation, object in question. x4 is possibly variable but may also be fixed by the environmwnt/context/situation. The equality should be exact in a model/in theory. If one is taking a linear approximation of a function which does not behave linearly with respect to x2, then caveats must be included (such as by using a tanru in order to specify approximation) or by separation (using be; "f approximates g, which is proportional [tartidu] to h, with error z, which is proportional [tartidu] to w"). x2 can be any of a fairly general class of nunvers, including a variable/unknown (with or without units) and a function (which need not be linear). Inverse or joint proportionality can simply be expressed using division and multiplication respectively. All terbri should be filled with either pure numbers (li; general) or quantified concepts (such as physical quantities).