Question

11) For an A.P. S12 =840, d = 10. Find the first term​

Answer 1

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→ Given : For an A.P. S12 =840, d = 10. Find the first term​ ?

→ Answer : First term = a = 15

→ Step-by-step explanation :

Sum of n terms in an AP is $S_n=\frac{n}{2}(2a+(n-1)d)$

Here , n = 12 , a = ? , d = 10 , S12=840

$S_{12}=\frac{12}{2} (2a+(12-1)10)\\\\=>840=6(2a+110)\\\\=>140=2a+110\\\\=>2a=30\\\\=>a=15$

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