Calculating a problem: 12 + 32 + 52 + 72 +. + 452 requires two solutions

Method 1
This is the arithmetic sequence, that is to say:
(the latter) minus (the former) = 20, and so on
The solution of summation is similar to trapezoidal formula: (upper bottom + lower bottom) XH / 2
(first item + last item) x items / 2
Then, first find the number of terms: (452 - 12) 20 + 1 = 23 -------- add 23 numbers in total
Sum = (12 + 452) * 23 / 2 = 5336
Method 2
As the number of items: (452 - 12) 20 + 1 = 23 -------- a total of 23 digits are added
Sum = (12 + 0 * 20) + (12 + 1 * 20) + (12 + 2 * 20) + (12 + 3 * 20) +. + (12 + 22 * 20)
23 from 0 to 22
＝12*23 + 20（0+1+2+3+.+22）
＝5336
Method 3
Subtract 12 for each item and 23 in total
Sum = 12 * 23 + (0 + 20 + 2 * 20 + 3 * 20 +. + 22 * 20)
＝12*23 + 20（0+1+2+3+.+22）
＝12*23 + 20（22*23/2）
＝5336

52 why is the probability of a pair of Zhang suoha calculated in this way?
The number of such hands is (13-choose-1)(4-choose-2)(12-choose-3)[(4-choose-1)]^3.If all hands are equally likely,the probability of a single pair is obtained by dividing by (52-choose-5).This probability is 0.422569.
Namely
13c1 4c2 * 12c3 * 4e3 / 52c5 = 0.422569
c: Combination
Forgive me for being stupid

Choose a pair first:
13C1, choose any one of the 13 cards
4C2, two of four colors
Choose three more cards that don't repeat
12C3, the remaining three cards choose three different points from 12
4 * 3, each point can have four different decors
Finally divide by the total number of possible combinations of cards
52c5

52.8-3x = 11.7 please explain the calculation method

52.8-3x=11.7
3x=52.8-11.7
3x=41.1
x=13.7

Xiaojuan lost the decimal point in divisor 2.81 when calculating a question. The result is 11.52, so the correct result should be ()

1152

Calculating a problem: 12 + 32 + 52 + 72 +. + 452 requires two solutions