Whats wrong with my quadratic formula?

Question

Collin Kerr 2016-4-19

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https://ww2.mathworks.cn/matlabcentral/answers/279722-whats-wrong-with-my-quadratic-formula

回答： Roger Stafford 2016-4-19

采纳的回答： James Tursa

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V_o = input('Velocity at launch = ');

Theta = input('Elevation angle at launch = ');

a=9.81; %Its a given

b=V_o*sind(Theta) %It makes the coding a bit less confusing.

c=0; %Also a given

    fprintf('The velocity for the launch is %d meters per second. \n', V_o)
    fprintf('The elevation angle for the launch is %d degrees. \n', Theta)
 Q1=(-b-sqrt(b^2-4*a*c))/2*a;
 disp(Q1)
 For some reason the code is not lining up with a calculators numbers. I was wondering if someone could explain what specifically is wrong with the quadratic formula and how could I be able to fix it?

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Answer 1

James Tursa 2016-4-19

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编辑：James Tursa 2016-4-19

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You need to enclose the denominator in parentheses:

Q1=(-b-sqrt(b^2-4*a*c))/(2*a);

The way you have it coded, since / and * have the same precedence the operations are performed left to right and MATLAB sees your current coded expression as:

Q1=((-b-sqrt(b^2-4*a*c))/2) * a;

Which is not what you wanted.

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Answer 2

Roger Stafford 2016-4-19

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Apparently you are trying to see how many seconds the object in question would take to return to earth. The velocity equation is:

v = -a*t+b

If you integrate with respect to t, you get

x = int(v,t) = -a*t^2/2+b*t

The solution is either t = 0 (the beginning) or t = b/(2*a) (at the end.) Your mistake was twofold. You had a positive value for a, and you didn't include the 1/2.