Check my working please (1 Viewer)

[powlmao]

(a)
A skyscraper of 110 floors is to be built. The first floor to be built will cost $3 million. The cost of building each subsequent floor will be $0.5 million more than the floor immediately below.
(i)
What will be the cost of building the 25th floor?
2
(ii)
What will be the cost of building all 110 floors of the skyscraper?
2


A)

Sum of 25 floors = n/2(2a + (n-1)d)
=25/2 (2(3) + (25-1) 1/2)
=225million


B)

Sum of all floors 110= n/2 ( 2a + (n-1)d)
=110/2 ( 2(3) + (110-1) 1/2)
=3327.5milllion


Is it correct?



I miss read the first part

T25= a +(n-1)d
= 3 + (25-1)1/2
= 3 + 12
=15million

 

[viraj30]

i think that u dont have to find the sum of the cost of 25 floors...its just the cost of that particular floor?? thats what i think but i can b wrong

 

[Carrotsticks]

For (A), the question asked how much to build the actual 25th floor, not 25 floorS

So you would be using the T_25 formula, not S_25.

 

[powlmao]

For (A), the question asked how much to build the actual 25th floor, not 25 floorS

So you would be using the T_25 formula, not S_25.

Oh derp. I mis-read the question. Other than that would it be right?

 

[RealiseNothing]

Oh derp. I mis-read the question. Other than that would it be right?

I remember doing this question, from memory, you got the total of all 110 floors correct.

 

[powlmao]

I remember doing this question, from memory, you got the total of all 110 floors correct.

Goodie. Its a 2011 question.