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Question Number 58373 by Tawa1 last updated on 22/Apr/19

Find the coefficient of  x^6   in   (2x + 1)^6  (x^2  + x + (1/4))^4

$$\mathrm{Find}\:\mathrm{the}\:\mathrm{coefficient}\:\mathrm{of}\:\:\mathrm{x}^{\mathrm{6}} \:\:\mathrm{in}\:\:\:\left(\mathrm{2x}\:+\:\mathrm{1}\right)^{\mathrm{6}} \:\left(\mathrm{x}^{\mathrm{2}} \:+\:\mathrm{x}\:+\:\frac{\mathrm{1}}{\mathrm{4}}\right)^{\mathrm{4}} \\ $$

Commented by maxmathsup by imad last updated on 24/Apr/19

we have (2x+1)^6 (x^2  +x+(1/4))^4  =2^6 (x+(1/2))^6 (x+(1/2))^8   =2^6 (x+(1/2))^(14)   =2^6  Σ_(k=0) ^(14)  C_(14) ^k  x^k  ((1/2))^(14−k)  =(2^6 /2^(14) ) Σ_(k=0) ^(14)  C_(14) ^k  2^k   x^k    so the coefficient  of x^6  is  a_6 =2^(−8)  C_(14) ^6   2^6  =(1/4) C_(14) ^6   =(1/4)  ((14!)/(6!8!))  .

$${we}\:{have}\:\left(\mathrm{2}{x}+\mathrm{1}\right)^{\mathrm{6}} \left({x}^{\mathrm{2}} \:+{x}+\frac{\mathrm{1}}{\mathrm{4}}\right)^{\mathrm{4}} \:=\mathrm{2}^{\mathrm{6}} \left({x}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{6}} \left({x}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{8}} \\ $$$$=\mathrm{2}^{\mathrm{6}} \left({x}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{14}} \:\:=\mathrm{2}^{\mathrm{6}} \:\sum_{{k}=\mathrm{0}} ^{\mathrm{14}} \:{C}_{\mathrm{14}} ^{{k}} \:{x}^{{k}} \:\left(\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{14}−{k}} \:=\frac{\mathrm{2}^{\mathrm{6}} }{\mathrm{2}^{\mathrm{14}} }\:\sum_{{k}=\mathrm{0}} ^{\mathrm{14}} \:{C}_{\mathrm{14}} ^{{k}} \:\mathrm{2}^{{k}} \:\:{x}^{{k}} \:\:\:{so}\:{the}\:{coefficient} \\ $$$${of}\:{x}^{\mathrm{6}} \:{is}\:\:{a}_{\mathrm{6}} =\mathrm{2}^{−\mathrm{8}} \:{C}_{\mathrm{14}} ^{\mathrm{6}} \:\:\mathrm{2}^{\mathrm{6}} \:=\frac{\mathrm{1}}{\mathrm{4}}\:{C}_{\mathrm{14}} ^{\mathrm{6}} \:\:=\frac{\mathrm{1}}{\mathrm{4}}\:\:\frac{\mathrm{14}!}{\mathrm{6}!\mathrm{8}!}\:\:. \\ $$

Answered by tanmay last updated on 22/Apr/19

(2x+1)^6 [(x+(1/2))^2 ]^4   (2x+1)^6 (x+(1/2))^8      or method  (2x+1)^6 [(x+(1/2))^2 ]^4   =2^6 (x+(1/2))^6 (x+(1/2))^8   =2^6 (x+(1/2))^(14)   =2^6 ×14c_8 ×x^6 ×((1/2))^8   =((14×13×12×11×10×9)/(6×5×4×3×2×2^2 ))  =((14×13×11×9)/(6×4))  =((7×13×11×3)/4)=((3003)/4)

$$\left(\mathrm{2}{x}+\mathrm{1}\right)^{\mathrm{6}} \left[\left({x}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{2}} \right]^{\mathrm{4}} \\ $$$$\left(\mathrm{2}{x}+\mathrm{1}\right)^{\mathrm{6}} \left({x}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{8}} \\ $$$$\: \\ $$$$\boldsymbol{{or}}\:\boldsymbol{{method}} \\ $$$$\left(\mathrm{2}\boldsymbol{{x}}+\mathrm{1}\right)^{\mathrm{6}} \left[\left(\boldsymbol{{x}}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{2}} \right]^{\mathrm{4}} \\ $$$$=\mathrm{2}^{\mathrm{6}} \left(\boldsymbol{{x}}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{6}} \left(\boldsymbol{{x}}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{8}} \\ $$$$=\mathrm{2}^{\mathrm{6}} \left(\boldsymbol{{x}}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{14}} \\ $$$$=\mathrm{2}^{\mathrm{6}} ×\mathrm{14}\boldsymbol{{c}}_{\mathrm{8}} ×\boldsymbol{{x}}^{\mathrm{6}} ×\left(\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{8}} \\ $$$$=\frac{\mathrm{14}×\mathrm{13}×\mathrm{12}×\mathrm{11}×\mathrm{10}×\mathrm{9}}{\mathrm{6}×\mathrm{5}×\mathrm{4}×\mathrm{3}×\mathrm{2}×\mathrm{2}^{\mathrm{2}} } \\ $$$$=\frac{\mathrm{14}×\mathrm{13}×\mathrm{11}×\mathrm{9}}{\mathrm{6}×\mathrm{4}} \\ $$$$=\frac{\mathrm{7}×\mathrm{13}×\mathrm{11}×\mathrm{3}}{\mathrm{4}}=\frac{\mathrm{3003}}{\mathrm{4}} \\ $$

Commented by Tawa1 last updated on 23/Apr/19

God bless you sir

$$\mathrm{God}\:\mathrm{bless}\:\mathrm{you}\:\mathrm{sir} \\ $$

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