quad2<-function(a1,b1,c1){
#求解一元二次方程的解a1*x^2+b1*x+c1=0
if(a1==0&&b1==0&&c1==0){
roots<-NA
}else if(a1==0&&b1==0){
roots<-NULL} else if(
a1==0){
roots<--c1/b1}else {
discrim<-b1^2-4*a1*c1
if(discrim>=0){
roots<-(-b1+c(1,-1)*sqrt(discrim))/(2*a1)
}else {roots<-NULL}
}
return(roots)
}