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Find the coefficient of x4 in the expansion of ((1)/(p) + px)7.

Mathematics

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Answer

p=365\mathbf{p=\frac{36}{5}}

Step-by-step solution

3 steps
  1. Find the coefficient of x4x^4 in the expansion of (1p+px)7\left(\frac{1}{p} + px\right)^7.

    The general term in the binomial expansion of (a+b)n(a+b)^n is given by Tr+1=(nr)an−rbrT_{r+1} = \binom{n}{r} a^{n-r} b^r.
    For (1p+px)7\left(\frac{1}{p} + px\right)^7, we have a=1pa = \frac{1}{p}, b=pxb = px, and n=7n = 7.
    We want the term containing x4x^4, so we set r=4r=4.

    T4+1=(74)(1p)7−4(px)4T_{4+1} = \binom{7}{4} \left(\frac{1}{p}\right)^{7-4} (px)^4 T5=(74)(1p)3p4x4T_5 = \binom{7}{4} \left(\frac{1}{p}\right)^3 p^4 x^4

    Calculate the binomial coefficient:

    (74)=7!4!(7−4)!=7!4!3!=7×6×53×2×1=35\binom{7}{4} = \frac{7!}{4!(7-4)!} = \frac{7!}{4!3!} = \frac{7 \times 6 \times 5}{3 \times 2 \times 1} = 35

    Substitute this value back into the term:

    T5=35⋅1p3⋅p4x4=35p4−3x4=35px4T_5 = 35 \cdot \frac{1}{p^3} \cdot p^4 x^4 = 35 p^{4-3} x^4 = 35p x^4

    The coefficient of x4x^4 in the first expansion is C1=35pC_1 = 35p.

  2. Find the coefficient of x4x^4 in the expansion of (12−2x)6\left(\frac{1}{2} - 2x\right)^6.

    For (12−2x)6\left(\frac{1}{2} - 2x\right)^6, we have a=12a = \frac{1}{2}, b=−2xb = -2x, and n=6n = 6.
    We want the term containing x4x^4, so we set r=4r=4.

    T4+1=(64)(12)6−4(−2x)4T_{4+1} = \binom{6}{4} \left(\frac{1}{2}\right)^{6-4} (-2x)^4 T5=(64)(12)2(−2)4x4T_5 = \binom{6}{4} \left(\frac{1}{2}\right)^2 (-2)^4 x^4

    Calculate the binomial coefficient:

    (64)=6!4!(6−4)!=6!4!2!=6×52×1=15\binom{6}{4} = \frac{6!}{4!(6-4)!} = \frac{6!}{4!2!} = \frac{6 \times 5}{2 \times 1} = 15

    Substitute this value back into the term:

    T5=15⋅14⋅16x4=15⋅4x4=60x4T_5 = 15 \cdot \frac{1}{4} \cdot 16 x^4 = 15 \cdot 4 x^4 = 60 x^4

    The coefficient of x4x^4 in the second expansion is C2=60C_2 = 60.

  3. Use the given relationship to find the value of pp.

    The problem states that the coefficient of x4x^4 in the first expansion is 192 more than the coefficient of x4x^4 in the second expansion.

    C1=C2+192C_1 = C_2 + 192

    Substitute the values of C1C_1 and C2C_2:

    35p=60+19235p = 60 + 192 35p=25235p = 252

    Divide by 35 to solve for pp:

    p=25235p = \frac{252}{35}

    Both 252 and 35 are divisible by 7:

    p=252÷735÷7=365p = \frac{252 \div 7}{35 \div 7} = \frac{36}{5}

    The value of pp is 365\frac{36}{5} or 7.2. This satisfies the condition p>0p > 0.

    The final answer is p=365\mathbf{p=\frac{36}{5}}.

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