реши

Solving for coefficient sum with y present

To find the sum of the coefficients of terms containing y in the expansion of (x + y − z)^11, we can use a common trick where all variables are set to 1. For the full polynomial, P(1,1,1) evaluates to 1. To focus on terms with y, we compute the total sum minus the sum from terms without y.

Verifying coefficient sum for terms with y

To find the sum of coefficients of terms containing y, I set y=0 and evaluate the polynomial. P(1,0,1) equals 0, so the sum of terms containing y is 1 (from the total sum of coefficients minus the sum from terms without y). This result seems correct, and I’ll confirm by double-checking via a combinatorial approach.

Thought for 20s