Question

If the nth term of an sequence is 3-2n. Find the sum of fifteen term

Answer 1

Answer:

below

Step-by-step explanation:

given that the nth term of an AP is [3-2n]

⇒T1= 3-2×1 = 1 =a ------> the first term

⇒T2= 3-2×2 = -1               {note that T2 ≠ a2}

∴  T2-T1= -1-1 = -2 ---------> a2

∴ a2 - a1 = -2 - 1= -3 ---->d = common difference

hence we have a and we have d and now we can find out the sum of its 15th term

hence we know that Sn= n÷2[2a+{n-1}d]

=15÷2[2×1+{15-1}×-3]

=15÷2[2+14×-3]

=15÷2[2-42}

=15÷2[-40]

=15÷2×-40

=15×-20

= -300

Answer 2

Answer:

Step-by-step explanation:

Step-by-step explanation:

given that the nth term of an AP is [3-2n]

⇒T1= 3-2×1 = 1 =a ------> the first term

⇒T2= 3-2×2 = -1               {note that T2 ≠ a2}

∴  T2-T1= -1-1 = -2 ---------> a2

∴ a2 - a1 = -2 - 1= -3 ---->d = common difference

hence we have a and we have d and now we can find out the sum of its 15th term

hence we know that Sn= n÷2[2a+{n-1}d]

=15÷2[2×1+{15-1}×-3]

=15÷2[2+14×-3]

=15÷2[2-42}

=15÷2[-40]

=15÷2×-40

=15×-20

= -300