Question

If the first term of an AP is 12 and the last term is 72, find the number of terms that give the sum 420.

Answer 1

Given- a=12
l=72
Sn=420
We know that,
Sn=n/2 [a+l]
So, 420=n/2 [12+72]
420=n/2 [84]
420=42n
So, 10=n or
n=10

Answer 2

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Here,

a = T1 = 12

l = an = 72

we need a formula which is

S = n/2 [ a + an ]

now

Sn = 420

=> n/2 [ a + an ] = 420

=> n/2 [ 12 + 72 ] = 420

=> n/2 × 84 = 420

=> 42n = 420

=> n = 420/42

=> n = 10

.°. n = 10


answer is 10

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