#include<stdio.h>
void main()
{
int a,c;
printf("------------------计算器-------------------\n");
printf("\t简易型<1>/科学型<2>/退出<3>\n");
scanf("%d",&a);
if(a==1)
{
    float num1,num2,num3;
    printf("加<4>/减<5>/乘<6>/除<7>/向上<8>/退出<9>\n");
    scanf("%d%f%f%f",&c,&num1,&num2,&num3);
     switch(c)
    {
        case'4':
            num3=num1+num2;
            printf("%f+%f=%f\n",num1,num2,num3);
        break;
        case'5':
            num3=num1-num2;
            printf("%f-%f=%f\n",num1,num2,num3);
        break;
        case'6':
            num3=num1*num2;
            printf("%f*%f=%f\n",num1,num2,num3);
        break;
        case'7':
            {
                if(num2==0)
                    printf("除数不能为0\n\n");
                else
                {
                    num3=num1/num2;
                    printf("%f/%f=%f\n",num1,num2,num3);
                }
            }
            break;
        default:
            printf("输入错误\n");
     }
}
else if(a==2)
    printf("正弦<10>/余弦<11>/正切<12>/x^y<13>/ln<14>\nlog<15>/n!<16>/1/x<17>立方<18>/平方<19>\n\t向上<20>/退出<21>\n");
}

scanf("%d%f%f%f",&c,&num1,&num2,&num3);//num3是算出来的，不是键盘输入的吧

最简单的改法是把case后面的引号去掉