What is the sum of all positive integers upto 990, which are divisible...
The positive integers divisible by 7 are 7, 14, 21,28, ... Among these, 14, 28, 42 ... are also divisible by 2.
The series of integers whose sum we want is: 7, 21, 35, ...
This is an arithmetic progression with the first term = 7 and the common difference = 14.
The last term in this progression is 987, which is 141 x 7. The nth term in an arithmetic progression is given by an = a1 + (n - 1 )d
987 = 7 + (n - 1 )14
994 = 14n
n = 71
= 35.5 x 994 = 35287
Hence, option 3.
Alternatively, The required sum should also be divisible by 7.
Options 2 and 4 are eliminated. 990/7 = 141.43
141 positive integers upto 990 are divisible by 7.
Of these, 71 are odd and 70 are even.
Sum of these 71 odd numbers is asked. This sum should also be odd.
So, option 1 can also be eliminated.
Hence option 3.
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What is the sum of all positive integers upto 990, which are divisible...
To find the sum of all positive integers up to 990 that are divisible by 7 and not by 2, we need to follow a step-by-step approach.
Step 1: Find the first number that satisfies the given conditions.
- The first positive integer that is divisible by 7 and not by 2 is 7 itself.
Step 2: Find the last number that satisfies the given conditions.
- We need to find the largest positive integer less than or equal to 990 that satisfies the given conditions.
- To find this, we divide 990 by 7 and take the integer part: 990 ÷ 7 = 141.
- Therefore, the largest positive integer less than or equal to 990 that is divisible by 7 is 141 x 7 = 987.
Step 3: Find the number of terms that satisfy the given conditions.
- To find the number of terms, we divide the last number (987) by the common difference (7) and add 1: (987 - 7) ÷ 7 + 1 = 141.
Step 4: Calculate the sum using the arithmetic progression formula.
- The sum of an arithmetic progression can be calculated using the formula: sum = (n/2)(2a + (n-1)d), where n is the number of terms, a is the first term, and d is the common difference.
- In this case, n = 141, a = 7, and d = 7.
- Substituting these values into the formula, we get: sum = (141/2)(2(7) + (141-1)(7)) = 71(14 + 140(7)) = 71(14 + 980) = 71(994) = 70,274.
Therefore, the sum of all positive integers up to 990 that are divisible by 7 and not by 2 is 70,274.
The correct answer is option C) 35,287.
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