A.y″+9y′=0 B.y″-9y′=0 C.y″+9y=0 D.y″-9y=0
A.f1(x)f′2(x)-f2(x)f′1(x)=0 B.f1(x)f′2(x)-f2(x)f′1(x)≠0 C.f1(x)f′2(x)+f2(x)f′1(x)=0 D.f1(x)f′2(x)+f2(x)f′1(x)≠0
A.y=cy1(x)+y2(x) B.y=y1(x)+c2y2(x) C.y=c[y1(x)+y2(x)] D.y=c1y(x)-y2(x)