Choosen As Best Answer

6.
a1 + a2 = 32
a3 + a4 = 30
假設 首項 = a、公差 = d
a + a + d = 32
a + 2d + a + 3d = 30
2a + d = 32 ...(1)
2a + 5d = 30 ...(2)
解聯立可得:a = 65/4, d = -1/2

第30期 a30 = a + 29d = 65/4 + 29×(-1/2) = 7/4

S30 = (65/4 + 7/4)×30÷2 = 18×15 = 270(萬)
再加上先付的 150(萬),總價 = 150 + 270 = 420(萬) #

8.
三個三個一組,120項共可分成 120 ÷ 3 = 40
即:0 + 3 + 6 + ... + a40
a40 = 0 + 39 × 3 = 117
總和 = (0 + 117) × 40 ÷ 2 = 2340 #

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