Topics of quadratic term theorem The coefficients of (2x ^ 2 + A / x ^ 3) ^ 10, x ^ 5 and x ^ 15 are the same I don't know what format the process should be written in

Topics of quadratic term theorem The coefficients of (2x ^ 2 + A / x ^ 3) ^ 10, x ^ 5 and x ^ 15 are the same I don't know what format the process should be written in

R + 1 is C10 (R) * (2x ^ 2) ^ (10-r) * (A / x ^ 3) ^ R = C10 (R) * 2 ^ (10-r) * a ^ R * x ^ (20-2r-3r)
Let 20-5r = 5, then r = 3
Let 20-5r = 15, then r = 1
Therefore, C10 (3) * 2 ^ (10-3) * a ^ 3 = C10 (1) * 2 ^ (10-1) * a
That is 120 * 2 ^ 7 * a ^ 3 = 10 * 2 ^ 9 * a
3a^3-a=0
a(3a^2-1)=0
If a is not equal to 0, then 3A ^ 2-1 = 0
So a = (+ / -) radical 3 / 3