Question

I am trying to align the following equations around the equal sign. What can I do? Thanks! I am using the AMSMath package

    $$mddot{x}_{1}-K(x_{2}+x_{12})+C(dot{x}_{12}+dot{x}_{2})+2Cdot{x}_{1} = fx_{1}$$
$$mddot{x}_{2}-K(x_{3}+x_{1})+C(dot{x}_{1}+dot{x}_{3})+2Cdot{x}_{2} = fx_{2}$$ 
$$mddot{x}_{3}-K(x_{4}+x_{2})+C(dot{x}_{2}+dot{x}_{4})+2Cdot{x}_{3} = fx_{3}$$

Answer 1

egin{align}
    mddot{x}_{1}-K(x_{2}+x_{12})+C(dot{x}_{12}+dot{x}_{2})+2Cdot{x}_{1} &= fx_{1} \
    mddot{x}_{2}-K(x_{3}+x_{1})+C(dot{x}_{1}+dot{x}_{3})+2Cdot{x}_{2} &= fx_{2} \
    mddot{x}_{3}-K(x_{4}+x_{2})+C(dot{x}_{2}+dot{x}_{4})+2Cdot{x}_{3} &= fx_{3}
end{align}

See the user s guide.

Answer 2

To make a long story short... the following idea of using eqnarray is kind of obvious, but turns out to be a really bad idea. eqnarray brings lots of issues which are solved in amsmath and related packages. Read Lars Madsen: Avoid eqnarray for the details.

egin{eqnarray*} %% Do avoid eqnarray if possible.
    x_1 & = & 1 \
    x_2 & = & 2 \
    x_3 & = & 3
end{eqnarray*}

(Answer revised with information from Arthur Reutenauer, Thomas and the Madsen paper linked to by Thomas.)

Answer 3

There are several packages to do what you want, one of the most widely used is the AMS align environment from the amsmath package.

Simplified extract from amsldoc.pdf:

egin{align} 
x&=y \ 
x’&=y’ \ 
x+x’&=y+y’ 
end{align}

The equations are aligned around the symbols that follow & (the equals signs, in this case).

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