Wrong root from quadratic equation calculator-CodePudding

I was wondering if someone could help me in this problem. So i tested the code but it didn't show the right answer below for equation result of x2 5x 6

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
        
double roots() {
    double a, b, c, x1, x2;
    cout << "Enter quadratic equation in order (a, b, c): \n";
    cin >> a >> b >> c;
    double discriminant = b * b - 4 * a * c;
    x1 = -b   sqrt(discriminant) / 2 * a;
    x2 = -b - sqrt(discriminant) / 2 * a;
    
    if (discriminant >= 0 && a > 1) {
        cout << "Your quadratic equation is " << a << "x^2   " << b << " x   " << c << '\n';
        cout << "x1 = " << x1 << '\n';
        cout << "x2 = " << x2 << '\n';
    }
    else if (a == 1) {
        cout << "Your quadratic equation is " << "x^2   " << b << " x   " << c << '\n';
        cout << "x1 = " << x1 << '\n';
        cout << "x2 = " << x2 << '\n';
    }
    else {
        cout << "Negative value returned from (b2 - 4ac), please try again!";
        exit(1);
    }
}
    
int main() {
    roots();
}

CodePudding user response：

You have the formula incorrect. Try this

x1 = (-b   sqrt(discriminant)) / (2 * a);
x2 = (-b - sqrt(discriminant)) / (2 * a);

Notice the extra parenthesis in order to put 2*a in the denominator and have it divide both b and the sqrt().

You also need to check if discriminant >= 0 before doing so, because if it is negative there is no root and the above lines are going to fail.

CodePudding user response：

Firstly, using namespace std was not used, so if you don't want to use it write std::court and std::cin. Secondly, the formula is b^2 -4ac so you need to put round brackets around b*b so that the answer is subtracted from -4ac. Then, you don't need to write else if for a==1 you can add it in the above condition as a>=1 and else put down a condition where discriminant is >=0 but a==0 which violates quadratic eq condition and you can write a cannot be equal to zero. Also, the main formula for x1 and x2 is wrong since the bracket should be applied around -b sqroot(discriminant) so that the answer is then divided by multiplication of (2*a). Otherwise, what happens is that first sqrt is divided by 2 then multiplied by a and then added to -b.

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
    
double roots() {
double a, b, c, x1, x2;
std::cout << "Enter quadratic equation in order (a, b, c): \n";
std::cin >> a >> b >> c;
double discriminant = (b * b)- (4 * a * c);
x1 = (-b   sqrt(discriminant)) / (2 * a);
x2 = (-b - sqrt(discriminant))/ (2 * a);

if (discriminant >= 0 && a >= 1) {
    std::cout << "Your quadratic equation is " << a << "x^2   " << b << " x 
  " << c << '\n';
    std::cout << "x1 = " << x1 << '\n';
     std::cout << "x2 = " << x2 << '\n';
}
else if (a==0){
     std::cout << "a cannot be zero!";
    exit(1);
}
else{
     std::cout << "Negative value returned from (b2 - 4ac), please try again!";
    exit(1);
}
}

int main() {
roots();
}