Q. Find the 4 th term in the expansion of (x−3)8 in simplest form.Answer:
Use Binomial Theorem: To find the 4th term in the expansion of (x−3)8, we will use the binomial theorem. The binomial theorem states that the kth term in the expansion of (a+b)n is given by C(n,k−1)⋅a(n−k+1)⋅b(k−1), where C(n,k) is the binomial coefficient "n choose k". For the 4th term, k=4.
Calculate Binomial Coefficient: First, we calculate the binomial coefficient C(8,4−1) which is C(8,3). This is equal to 3!×(8−3)!8!, where “!“ denotes factorial.
Simplify Factorials: Calculating the factorial values, we get 8!=8×7×6×5×4×3×2×1, 3!=3×2×1, and (8−3)!=5!=5×4×3×2×1.
Calculate Binomial Coefficient: Now, we simplify the binomial coefficient C(8,3)=3!×5!8!=3×2×18×7×6=56.
Calculate Powers of a and b: Next, we calculate the remaining parts of the term: a(n−k+1)=x(8−4+1)=x5 and b(k−1)=(−3)(4−1)=(−3)3.
Simplify Powers: Now, we simplify (−3)3=−27.
Multiply Coefficients: Finally, we multiply the binomial coefficient by the powers of a and b to get the 4th term: 56×x5×(−27).
Final Result: Multiplying 56 by −27, we get −1512. So, the 4th term in the expansion is −1512×x5.
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