Question

ICBSE
In an AP. If the first term is 22. the common difference is -4 and the sum to n terms is 64,find n.​

Answer 1

$an =( a + n - 1) \times d$

this formula is used in this question....

Answer 2

Answer:

a = 24

d = -4

Sn = 64

= n/2 {2a + (n- 1_d) = 64

= n/2 {2*22 + (n - 1) * (-4)} = 64

= n/2 {44 - 4n + 4} = 64

= n*4 {11-n + 1} = 64*2

= 11n - n sq. + n = 128/4

= - n sq. + 11n + n = 32

= - n sq. + 12n - 32 = 0

= n sq. - 12n + 32 = 0

= n sq. - 4n + 8n + 32 = 0

= n (n - 4) - 8 ( n - 4) = 0

= (n-4) (n-8) = 0

n = 4 or n = 8

Hope it helps.

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