C語言練習-if語句

void quadratic_equation(void)
{


    float a,b,c,root1,root2,d,vr1,vr2;
    printf("Enter a,b,c:");
    scanf("%f %f %f",&a,&b,&c);
    if(!a)
    {
        if(!b)
        {
            if(!c)
                printf("any root");
            else
                printf("no root!");
        }

        printf("only one root root = %f",-c/b);
    }
    else
    {
        d=b*b-4*a*c;
        if(d>=0)
        {
            root1 = (-b+sqrt(d))/2*a;
            root2 = (-b-sqrt(d))/2*a;
            printf("root1 = %f, root2 = %f",root1,root2);
        }
        else
        {
            vr1=-b/2*a;
            vr2=sqrt(-d)/2*a;
            printf("root1 =%6.2f  +%6.2fi\n",vr1,vr2);
            printf("root2 =%6.2f  -%6.2fi",vr1,vr2);
        }
    }
}

2.Personal_Tax_Calcuator

void P_Tax(void)
{
    float dSalary,dTax=0,dNetIncome=0,dRate = 0;
    printf("Enter your salary: ");
    scanf("%f", &dSalary);
    float a=dSalary-3500;
    if(a<0)
        dRate = 0, dNetIncome = 0;
    else if(a<1500)
        dRate = 0.03, dNetIncome = 0;
    else if(a<4500)
        dRate = 0.10, dNetIncome = 105;
    else if(a<9000)
        dRate = 0.20, dNetIncome = 555;
    else if(a<35000)
        dRate = 0.25, dNetIncome = 1005;
    else if(a<55000)
        dRate = 0.30, dNetIncome = 2755;
    else if(a<80000)
        dRate = 0.35, dNetIncome = 5505;
    else
        dRate = 45, dNetIncome = 13505;

    dTax = a*dRate - dNetIncome;


    printf("your tax is %.2f , \nafter tax income is %.2f.\n", dTax, dSalary-dTax);
	printf("Thanks for your using!\n");
}

void tri_angle(void)
{
    float x,y;
    int flag=1;
    scanf("%f",&x);
    if(x>=0)
        y=(sin(x/180*pi)+flag*cos(x/180*pi))/2;
    else
    {
        flag=!flag;
        y=(sin(x/180*pi)+flag*cos(x/180*pi))/2;
    }
    printf("y is %f",y);
}