Choosen As Best Answer

(180 - ∠A) - (90 - ∠B)
= 90 - ∠A + ∠B
= 90 - (∠A - ∠B) = 40 #

\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

a1 + 21 = a2 + 20 = ... = a10 + a12 = 中間項 a11 的兩倍

每組總和 = 1290 ÷ 1.5(一組 + 中間項的半組) = 860
前三項 + 後三項 = 860 × 3 = 2580
2580 - 2370 = 210 #

(看不懂我再補充)

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