How to neglect the terms containing second and higher powers of parameter in a function

Question

I want to neglect the terms containing second and higher powers of a from some equations. Since the equations are lengthy expressions, I was using Mathematica to neglect these terms. I have tried but I was unable. Can anyone please help me in this regard?

For example, for the following equation, I want to remove the terms containing a^2 and higher powers than 2.

(2 r^4 (4 a^2 + (27 c13)/(16 (-1 + c13)) + r^2 (Q + (-2 + r) r))^3 ((
    81 c13)/(-1 + c13) + 16 r^2 (2 Q + (-3 + r) r) + 
    24 a Sqrt[-((54 c13)/(-1 + c13)) + 16 r^2 (-Q + r)])), (-((
    a^2 (-16 r^2 (Q + (-2 + r) r) + 
       c13 (27 + 16 Q r^2 - 32 r^3 + 16 r^4))^2)/(
    4 (-1 + c13)^2 r^2)) + 
   16 a^3 Sqrt[1 + (27 c13)/(16 (-1 + c13) r^4) + (Q - 2 r)/r^2]
     Sqrt[(27 c13)/(-1 + c13) + 
     16 r^2 (Q + (-2 + r) r)] (-12 a + 
      Sqrt[-((54 c13)/(-1 + c13)) + 16 r^2 (-Q + r)]) + (
   4 a^2 ((27 c13)/(16 (-1 + c13)) + r^2 (Q + (-2 + r) r)) (-12 a + 
      Sqrt[-((54 c13)/(-1 + c13)) + 16 r^2 (-Q + r)])^2)/
   r^2 + (((27 c13)/(16 (-1 + c13)) + r^2 (Q + (-2 + r) r)) (4 a^2 + (
      27 c13)/(16 (-1 + c13)) + r^2 (Q + (-2 + r) r)) (-12 a + 
      Sqrt[-((54 c13)/(-1 + c13)) + 16 r^2 (-Q + r)])^2)/
   r^2)/((4 a^2 + (27 c13)/(16 (-1 + c13)) + 
     r^2 (Q + (-2 + r) r))^2 ((81 c13)/(-1 + c13) + 
     16 r^2 (2 Q + (-3 + r) r) + 
     24 a Sqrt[-((54 c13)/(-1 + c13)) + 16 r^2 (-Q + r)]))  

Answer 1

You seem to be looking for the linear approximation. That is the first two terms of the Taylor series. For example:

Normal[Series[(a + x) (a - x) (a - 2 x), {a, 0, 1}]]

yields

-a x^2 + 2 x^3