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A string is divided into 55 parts with lengths forming a geometric sequence. If the shortest string is 1616 cm and the longest string is 8181 cm, then the original length of the string is...

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Q. A string is divided into 55 parts with lengths forming a geometric sequence. If the shortest string is 1616 cm and the longest string is 8181 cm, then the original length of the string is...
  1. Given Information: We are given that the string is divided into 55 parts with lengths forming a geometric sequence. The shortest part is 1616 cm and the longest part is 8181 cm. In a geometric sequence, each term after the first is found by multiplying the previous term by a constant called the common ratio (rr). We need to find the common ratio and then use it to find the sum of the sequence, which will give us the original length of the string.
  2. Equation for Longest String: Let's denote the shortest string as aa (the first term of the geometric sequence). The longest string will be a×r(n1)a \times r^{(n-1)}, where nn is the number of terms, which is 55 in this case. We can set up the equation for the longest string as follows:\newlinea×r(51)=81cma \times r^{(5-1)} = 81 \, \text{cm}\newlineSince we know the shortest string is 16cm16 \, \text{cm}, we can substitute aa with 16cm16 \, \text{cm}:\newline16×r4=8116 \times r^4 = 81
  3. Finding Common Ratio: To find the common ratio rr, we need to solve the equation:\newline16r4=8116 \cdot r^4 = 81\newlineDivide both sides by 1616 to isolate r4r^4:\newliner4=8116r^4 = \frac{81}{16}\newliner4=5.0625r^4 = 5.0625\newlineNow, take the fourth root of both sides to find rr:\newliner=(5.0625)14r = (5.0625)^{\frac{1}{4}}\newliner1.5r \approx 1.5
  4. Finding Sum of Geometric Sequence: Now that we have the common ratio, we can find the sum of the geometric sequence using the formula for the sum of the first nn terms of a geometric sequence: Sn=a×(1rn)/(1r)S_n = a \times (1 - r^n) / (1 - r) where SnS_n is the sum of the first nn terms, aa is the first term, rr is the common ratio, and nn is the number of terms. We have a=16a = 16 cm, r1.5r \approx 1.5, and n=5n = 5.
  5. Finding Sum of Geometric Sequence: Now that we have the common ratio, we can find the sum of the geometric sequence using the formula for the sum of the first nn terms of a geometric sequence: Sn=a1rn1rS_n = a \cdot \frac{1 - r^n}{1 - r} where SnS_n is the sum of the first nn terms, aa is the first term, rr is the common ratio, and nn is the number of terms. We have a=16a = 16 cm, r1.5r \approx 1.5, and n=5n = 5. Let's plug the values into the sum formula: S5=1611.5511.5S_5 = 16 \cdot \frac{1 - 1.5^5}{1 - 1.5} S5=1617.5937511.5S_5 = 16 \cdot \frac{1 - 7.59375}{1 - 1.5} S5=166.593750.5S_5 = 16 \cdot \frac{-6.59375}{-0.5} S5=1613.1875S_5 = 16 \cdot 13.1875 \[S_5 = \(211\)\) cm

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