Question

in an A.P. Sn=9360, n=12, d=-40, a=?, t12=?​

Answer 1

a=1040 and t12=600
sn=n/2(2a+(n-1)d)
by substituting the given values we get a equal to 1040 and t12= a+11d so t12=600

Answer 2

Step-by-step explanation:

Sn=9360

n=12

d=40

S12=n/2[a+(n-1)d]

$9360 = 12 \div 2(a +( 12 - 1)( - 40) \\ 9360 = 6(a + (11) (- 40) \\ 9360 \div 6 = a - 440 \\ 1560 = a - 440 \\ 1560 + 440 = a \\ a = 2000$

t12=a+11d

= 2000+11×(-40)

=2000-440

=1560

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