You are correct that the lower limit r=5 is crucial for part (b). Here's how it was used: 1. First Term: The first term of the series is found by substituting r=5 into the expression (70 + 4r). T₅ = 70 + 4(5) = 70 + 20 = 90; 2. Number of Terms: For a summation from r=a to r=b, the number of terms is b − a + 1. In this case, a=5 and b=n. Number of terms = n − 5 + 1 = n − 4 These two values (the first term and the number of terms) were then used in the sum formula for an arithmetic series.

The lower limit r=5

October 1, 2026Mathematics

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You are correct that the lower limit $r=5$ is crucial for part (b).

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Here's how it was used:

First Term
The first term of the series is found by substituting $r=5$ into the expression $(70 + 4r)$ .
$T_5 = 70 + 4(5) = 70 + 20 = 90$
Number of Terms
For a summation from $r=a$ to $r=b$ , the number of terms is $b - a + 1$ . In this case, $a=5$ and $b=n$ .
$\text{Number of terms} = n - 5 + 1 = n - 4$
These two values (the first term and the number of terms) were then used in the sum formula for an arithmetic series.

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