Fundamental set of solutions

Theorem 3.2 is a statement of the superposition principle [1,2,4], which is also applicable to higher-order linear differential equations. The two solutions yi and y2 form what is called a fundamental set of solutions for Equation 3.10. 

In general, two solutions yi and y2 of Equation 3.10 are said to form a fundamental set of solutions if every solution of Equation 3.10 can be expressed as a linear combination of yi and y2- In particular... 

To use this method we must know a fundamental set of solutions (for the second-order case and yf) for the homogeneous equation. 

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