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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. ### how to calculate total surface area？?

Frist I let the length be l, height be h, and the width be w. Then there will... the basic way to calculate)  For more information, just search in Wikipedia and you will get the answer (also Google ...

分類：科學及數學 > 數學 2021年01月21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. ### 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

5. ### 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

6. ### 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

7. ### 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

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