686. Запишите в виде многочлена выражение: \( \begin{array}{ll}\text { а) }(x+1)(x+2)(x+3) ; & \text { б) }(a-1)(a-4)(a+5) .\end{array} \)

Simplify the expression by following steps: - step0: Calculate: \(\left(a-1\right)\left(a-4\right)\left(a+5\right)\) - step1: Multiply the terms: \(\left(a^{2}-5a+4\right)\left(a+5\right)\) - step2: Apply the distributive property: \(a^{2}\times a+a^{2}\times 5-5a\times a-5a\times 5+4a+4\times 5\) - step3: Multiply the terms: \(a^{3}+5a^{2}-5a^{2}-25a+4a+20\) - step4: Collect like terms: \(a^{3}-21a+20\) Expand the expression \( (x+1)(x+2)(x+3) \) Simplify the expression by following steps: - step0: Calculate: \(\left(x+1\right)\left(x+2\right)\left(x+3\right)\) - step1: Multiply the terms: \(\left(x^{2}+3x+2\right)\left(x+3\right)\) - step2: Apply the distributive property: \(x^{2}\times x+x^{2}\times 3+3x\times x+3x\times 3+2x+2\times 3\) - step3: Multiply the terms: \(x^{3}+3x^{2}+3x^{2}+9x+2x+6\) - step4: Add the terms: \(x^{3}+6x^{2}+11x+6\) Запишем выражения в виде многочленов: а) \( (x+1)(x+2)(x+3) = x^{3} + 6x^{2} + 11x + 6 \) б) \( (a-1)(a-4)(a+5) = a^{3} - 21a + 20 \)