(9x-9) △ 3 = 5.4 (checking calculation) to solve the equation

(9x-9) △ 3 = 5.4 (checking calculation) to solve the equation

(9x-9)÷3=5.4
9x-9=5.4×3
9x-9=16.2
9x=16.2+9
9x=25.2
x=2.8
checking calculation:
(9×2.8-9)÷3=（25.2-9）÷3=16.2÷3=5.4
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How to solve the equation of 9x = 81, and his checking calculation, as well as the checking calculation of X △ 0.5 = 6 and X △ 13 = 0.8

9x = 81, then x = 81 △ 9, x = 9 checking calculation: substituting x = 9 into the original equation, then 9 × 9 = 81, so x = 9 is the solution of the original equation. X △ 0.5 = 6, then x = 0.5 × 6, x = 3 checking calculation: substituting x = 3 into the original equation, then 3 △ 0.5 = 6, so x = 3 is the solution of the original equation. X △ 13 = 0.8, then x = 13 × 0.8, x = 10.4 checking calculation: substituting x = 10.4 into the original equation

Calculate 1 + 2 / 1 + 1 + 2 + 3 / 1 to 1 + 2 + 3 + 4 1 / n

Because 1 + 2 +... + n = n (n + 1) / 2
So 1 / (1 + 2 +... + n) = 2 / [n (n + 1)] = 2 [1 / n-1 / (n + 1)]
So 1 / (1 + 2) + 1 / (1 + 2 + 3) +... + 1 / (1 + 2 +... + n)
=2[1/2-1/3]+2[1/3-1/4]+...+2[1/n-1/(n+1)]
=2(1/2-1/(n+1))
=(n-1)/(n+1)