Skill: Apply the binomial theorem

Explanation and Free Practice Resources

The binomial theorem represents (a+b)n(a+b)^n, for a nonnegative integer nn, as ∑k=0n(nk)an−kbk\sum_{k=0}^{n}\binom{n}{k}a^{n-k}b^k, enabling accurate expansion, identification of particular terms or coefficients, and interpretation of (nk)\binom{n}{k} as the number of ways to choose which factors contribute bb. It connects algebraic expansion with combinations and binomial probability; noninteger or negative exponents and infinite-series generalizations are outside this scope.

Detailed Explanation: Apply the binomial theorem

To expand a power of a binomial, use the binomial theorem:

(a+b)n=∑k=0n(nk)an−kbk(a+b)^n=\sum_{k=0}^{n}\binom{n}{k}a^{n-k}b^k

Here, (nk)\binom{n}{k} gives the coefficient, and kk tells how many factors contribute bb.

Example: Expand (2x−3)4(2x-3)^4

Identify the parts:

  • a=2xa=2x
  • b=−3b=-3
  • n=4n=4

Substitute into the theorem:

(2x−3)4=(40)(2x)4+(41)(2x)3(−3)+(42)(2x)2(−3)2+(43)(2x)(−3)3+(44)(−3)4(2x-3)^4 =\binom{4}{0}(2x)^4 +\binom{4}{1}(2x)^3(-3) +\binom{4}{2}(2x)^2(-3)^2 +\binom{4}{3}(2x)(-3)^3 +\binom{4}{4}(-3)^4

Use the binomial coefficients:

(40)=1,(41)=4,(42)=6,(43)=4,(44)=1\binom{4}{0}=1,\quad \binom{4}{1}=4,\quad \binom{4}{2}=6,\quad \binom{4}{3}=4,\quad \binom{4}{4}=1

Now simplify each term:

(2x−3)4=1(2x)4+4(2x)3(−3)+6(2x)2(−3)2+4(2x)(−3)3+1(−3)4=16x4−96x3+216x2−216x+81\begin{aligned} (2x-3)^4 &=1(2x)^4+4(2x)^3(-3)+6(2x)^2(-3)^2\\ &\quad+4(2x)(-3)^3+1(-3)^4\\ &=16x^4-96x^3+216x^2-216x+81 \end{aligned}

Therefore,

(2x−3)4=16x4−96x3+216x2−216x+81\boxed{(2x-3)^4=16x^4-96x^3+216x^2-216x+81}

The signs alternate here because b=−3b=-3, so odd powers of bb are negative.

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Binomial Theorem - Polynomial with Two Integers and Triangle to Expanded Polynomial


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