Respuesta :

Given expression is (a+b)^8 and we need to find the 5th term of this series.

Explanation -

The general binomial expression is given as

[tex](x+y)^n=^nC_0x^ny^0+^nC_1x^{n-1}y^1+^nC_2x^{n-2}y^2+............upto\text{ x}^0[/tex]

So the given expression can be written as

[tex](a+b)^8=^8C_0a^8b^0+^8C_1a^7y^1+^8C_2a^6b^2+^8C_3a^5b^3+^8C_4a^4b^4+...[/tex]

So the 5th term will be

[tex]\begin{gathered} 5th\text{ term = }^8C_4a^4b^4 \\ The\text{ general comb}\imaginaryI\text{nat}\imaginaryI\text{on formula }\imaginaryI\text{s = }^nC_r=\frac{n!}{(n-r)!r!} \end{gathered}[/tex]

Then we have

[tex]\begin{gathered} 5th\text{ term = }\frac{8!}{(8-4)!4!}a^4b^4 \\ 5th\text{ term = }\frac{8\times7\times6\times5}{4!}a^4b^4 \\ 5th\text{ term = }\frac{8\times7\times6\times5}{4\times3\times2\times1}a^4b^4=2\times7\times5\times a^4b^4=70a^4b^4=70(ab)^4 \end{gathered}[/tex]So the final answer is 70(ab)^4

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