A * B is 24. A * C is 36. A * C is 54. How much is a + B + C. a.23.b.21.c.19.d.17

A * B is 24. A * C is 36. A * C is 54. How much is a + B + C. a.23.b.21.c.19.d.17

AB multiplied by AC in division BC
It's the square of a = 16
So a equals plus or minus four
From ab = 24, AC = 36, BC = 54
B equals plus or minus 6
C equals plus or minus 9
A + B + C = 19 or - 19

In the following four groups of numbers, () has the maximum value of a, (1 / 12 + 1 / 13) * 20b, (1 / 17 + 1 / 19) * 30C, (1 / 23 + 1 / 25) * 40d, (1 / 35 + 1 / 39) * 60
A simple method is also needed

Four formulas can be compared as follows: ① (1 / 12 + 1 / 13) × 20 = (1 / 72 + 1 / 78) × 120; ② (1 / 17 + 1 / 19) × 30 = (1 / 68 + 1 / 76) × 120; ③ (1 / 23 + 1 / 25) × 40 = (1 / 69 + 1 / 75) × 120; ④ (1 / 35 + 1 / 39) × 60 = (1 / 70 + 1 / 78) × 120

1*3+3*5+5*7+...17+19

Let Sn = 1 * 3 + 3 * 5 +... + 17 * 19 + (2n-1) * (2n + 1) +
General term an = (2n-1) * (2n + 1) = 4N ^ 2-1
sn=4(1^2+2^2+3^2+...+N^2)-n
There are 8 terms in the above formula, S8 = 4 (1 + 2 ^ 2 +... + 8 ^ 2) - 8