Question

What are the coefficients for the binomial expansion of (a b)3? 1, 4, 6, 4, 1 1, 1 1, 2, 1 1, 3, 3, 1

Answer 1

The coefficients for the binomial expansion of (a + b)³ are 1, 3, 3, 1.

We have,

The binomial expansion (a + b)³,

So,

Now,

We know that,

Formula for binomial expansion,

i.e.

(u + v)ⁿ = ⁿC₀ · uⁿ · v⁰ + ⁿC₁ · uⁿ⁻¹ · v¹ + ⁿC₂ · uⁿ⁻² · v² + ⁿC₃ · uⁿ⁻³ · v³

Now,

According to the question,

Here,

u = a,

v = b,

And, n =3

Now,

Putting values,

i.e.

(a + b)³ = ³C₀ · a³ · b⁰ + ³C₁ · a³⁻¹ · b¹ + ³C₂ · a³⁻² · b² + ³C₃ · a³⁻³ · b³

We get,

(a + b)³ = ³C₀ · a³ · b⁰ + ³C₁ · a² · b¹ + ³C₂ · a¹ · b² + ³C₃ · a⁰ · b³

Now,

Using combination formula,

$^{n} C_{r} = \frac{n!}{r!(n-r)!}$

On solving we get,

(a + b)³ = a³ + 3 a² b + 3 a b² + b³

So,

Now,

The coefficients of :

a³ = 1,

3a²b = 3,

3ab² = 3,

b³ = 1,

So,

The coefficients of the given binomial expansion are 1, 3, 3, 1.

Hence we can say that the coefficients for the binomial expansion of (a + b)³ are 1, 3, 3, 1.

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