Arithmetic sequence, help?

[AndovarEdoras]

Could someone explain to me how to do the following (in steps):

 

(This is a question from my exam)

-Knowing that the third term is 10 and the fifth term is 16.

a) What's the first term?

b) What's the common difference?

c) What's the sum of the first 20 terms?

[Guest]

OK, are you sure the  questions are in order?  cause what i did is ---

 

STEP (1) find out the common difference ---- (16-10)/2  = 3   --- --(Question b)

STEP (2) use the arithmetic equation -- a + (n-1)(d) = Un    so a +( 3 - 1 )(3) = 10 , a = 4 (question a)

STEP (3) use the sum of arithmetic equation --- plug all the numbers. Sn = n/2(2u1+(n-1)d)  =  20/2 * ( 2*4 + (20-1)3) = 650. 

 

I hope it is the right answer and just send me message if i i didn't explain it fairly well. I am not a good teacher.

 

Good luck

[AndovarEdoras]

OK, are you sure the  questions are in order?  cause what i did is ---

 

STEP (1) find out the common difference ---- (16-10)/2  = 3   --- --(Question b)

STEP (2) use the arithmetic equation -- a + (n-1)(d) = Un    so a +( 3 - 1 )(3) = 10 , a = 4 (question a)

STEP (3) use the sum of arithmetic equation --- plug all the numbers. Sn = n/2(2u1+(n-1)d)  =  20/2 * ( 2*4 + (20-1)3) = 650. 

 

I hope it is the right answer and just send me message if i i didn't explain it fairly well. I am not a good teacher.

 

Good luck

 

 

Yes thats the order the questions were in. I also got those answers but i was unsure if i even did it correctly. Just like you, i solved the difference before the first term.

[Guest]

 

OK, are you sure the  questions are in order?  cause what i did is ---

 

STEP (1) find out the common difference ---- (16-10)/2  = 3   --- --(Question b)

STEP (2) use the arithmetic equation -- a + (n-1)(d) = Un    so a +( 3 - 1 )(3) = 10 , a = 4 (question a)

STEP (3) use the sum of arithmetic equation --- plug all the numbers. Sn = n/2(2u1+(n-1)d)  =  20/2 * ( 2*4 + (20-1)3) = 650. 

 

I hope it is the right answer and just send me message if i i didn't explain it fairly well. I am not a good teacher.

 

Good luck

 

 

Yes thats the order the questions were in. I also got those answers but i was unsure if i even did it correctly. Just like you, i solved the difference before the first term.

 

 

OH 

you can put those into simultaneous equation 

for question(a) ---  a + (3-1)(d) = 10 and a + (5-1)(d)    = 16, than rearranges it ,  take d as the subject and it will bring you to the answer a =4 

Edited January 27, 2015 by Guest

[AndovarEdoras]

 

 

OK, are you sure the  questions are in order?  cause what i did is ---

 

STEP (1) find out the common difference ---- (16-10)/2  = 3   --- --(Question b)

STEP (2) use the arithmetic equation -- a + (n-1)(d) = Un    so a +( 3 - 1 )(3) = 10 , a = 4 (question a)

STEP (3) use the sum of arithmetic equation --- plug all the numbers. Sn = n/2(2u1+(n-1)d)  =  20/2 * ( 2*4 + (20-1)3) = 650. 

 

I hope it is the right answer and just send me message if i i didn't explain it fairly well. I am not a good teacher.

 

Good luck

 

 

Yes thats the order the questions were in. I also got those answers but i was unsure if i even did it correctly. Just like you, i solved the difference before the first term.

 

 

OH 

you can put those into simultaneous equation 

for question(a) ---  a + (3-1)(d) = 10 and a + (5-1)(d)    = 16, than rearranges it ,  take d as the subject and it will bring you to the answer a =4 

 

 

I used the simultaneous equation to solve it like this. (I had 2-3min left on the clock before the exam ended to do this entire task, so i panicked and did the first thing that seemed logical to do..)