In all these cases, {{mvar|y}} is an unknown function of {{mvar|x}} (or of <math>x_1</math> and <math>x_2</math>), and {{mvar|f}} is a given function. | In all these cases, {{mvar|y}} is an unknown function of {{mvar|x}} (or of <math>x_1</math> and <math>x_2</math>), and {{mvar|f}} is a given function. |