a) \( \sin ^{4} x+\cos ^{2} x \cdot \sin ^{2} x-\sin ^{2} x \)

First, let's simplify the expression step by step: 1. Start with the original expression: \[ \sin^4 x + \cos^2 x \cdot \sin^2 x - \sin^2 x. \] 2. Notice that \( \cos^2 x = 1 - \sin^2 x \). We can substitute this into our expression: \[ \sin^4 x + (1 - \sin^2 x) \cdot \sin^2 x - \sin^2 x. \] 3. Distributing \( \sin^2 x \) gives: \[ \sin^4 x + \sin^2 x - \sin^4 x - \sin^2 x. \] 4. Combining like terms, we see that the \(\sin^4 x\) terms and the \(\sin^2 x\) terms cancel out: \[ 0. \] Thus, the simplified form of the expression is \(0\).