Each number in the sum is even, so we can remove a factor of 2.
2 + 4 + 6 + 8 + ... + 78 + 80 = 2 (1 + 2 + 3 + 4 + ... + 39 + 40)
Use whatever technique you used in (a) and (b) to compute the sum
1 + 2 + 3 + 4 + ... + 39 + 40
With Gauss's method, for instance, we have
S = 1 + 2 + 3 + ... + 38 + 39 + 40
S = 40 + 39 + 38 + ... + 3 + 2 + 1
2S = (1 + 40) + (2 + 39) + ... + (39 + 2) + (40 + 1) = 40×41
S = 20×21 = 420
Then the sum you want is 2×420 = 840.
Answer:

Step-by-step explanation:
This expression was kind of trial and error for me seeing as I'm not really good at logic, but this is pretty basic. There are three possible places the parentheses can be placed (assuming each pair holds 2 terms);
The answer to the first expression would be 45, which is not equal to 11.
The answer to the second expression would be 15, which is also not equal to 11.
The answer to the last expression is 11, so that is the correct answer.
Hope this helped :D
- Shay
Answer:
Surface area = 6l^2
Volume = l^3
If total surface area increased by 2, then the length increased by 
Volume then is increased by (
l)^3 = 2
l^3
1 :2 
Which is not an option in the question so this question clearly has a problem.
First of all It's supposed to be "then" not "than," so I'm not sure who teaches math without even knowing this basic grammar.
Second of all, if the answer is 8:1 the question should be "If <u>each sides</u> of a cube is doubled" not "the lateral surface area." Come to think of it what even is lateral surface area of a cube if all sides of the cube is supposed to be same.
Or, if the answer is 2:1 then, it should be If height lateral surface area of a cube is doubled <u>by doubling the height and without changing the length and width</u>" But, then the the shape would no longer be a cube.
Who even wrote this garbage question what.
To determine the answer of Part A draw the equilateral triangle and the to determine the coordinates of of the third charge use that triangle.
To calculate the gravitational field strength in part B from each of the charges use the following equation.
E=kcq/r2
If you would add those values then you can use the symmetry about the y axis to make the vector addition a litter easier.<span />