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1. ### How to prove cos(3pi/5)=((1-sqrt5)/4)?

昨晚的回答不知何故又被隱了, 簡單回答吧. 令 x = cos(3π/5) 則 cos(9π/5) = 4x^3 - 3x cos(6π/5) = 2x^2 - 1 cos(9π/5) + cos(6π/5) = 0 即      4x^3 + 2x^2 - 3x - 1       =  (x+1)(4x^2 - 2x - 1 ) = 0. 因 π/2 < 3π/5 < π, 故     0 > cos(3π/5) > -1 故 cos(3π/5) = (1-√5)/4.

分類：科學及數學 > 數學 2021年01月14日

2. ### Maths problem:為何答案是C?

...2(π - 1) cm³　　D.　(π - 2) cm³ Solution： Let r be the radius of a identical circle. In the picture 1, A perimeter of an identical circle = the perimeter of the shaded...

分類：科學及數學 > 數學 2020年05月25日

3. ### Maths problem: i don't know how to do (c), it's so difficult, thanks?

...4 = 41 Therefore, P(black) = 41/48. (c) Let the required number be n. After...black balls, there are (48 - n) balls in total where (41 - n) are black. Therefore...

分類：科學及數學 > 數學 2020年09月13日

4. ### Maths problem, how to do, thanks?

(a) Let height of a larger cylinder = H base area of a larger... As the 2 larger cylinders are recast in 20 smaller cylinders,  2πR²H = 120πr²...

分類：科學及數學 > 數學 2020年07月22日

5. ### Maths problem: how to do, thanks?

... paper II)    + P(Fail in paper I) P(Pass paper II) ...9) (1 - 2/3) (2/3) + (1 - 7/9) (2/3) (7/9) = 70/243 (bi) Let A be the event A that Julia ...

分類：科學及數學 > 數學 2020年06月11日

6. ### 數學功課求解!!!!?

Question： In ΔACD, AC = AD. B and ... = BC = 40 ...... ( opp. sides of // gram. ) Let G be the mid-pt of DF. ∵ ΔEFG...

分類：科學及數學 > 數學 2020年08月05日

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