Question

the eighth term of an arithmetic sequence is 40 calculate the sum of first 15 terms​

Answer 1

Answer:

600

Step-by-step explanation:

8th term of an ap

$> > a + (n - 1)d$

$= acc \: to \: questions \: value$

$= > a + (8 - 1)d = 40 >$

$= > a + 7d = 40.............(1)$

$sum = \frac{n}{2} (2a + (n - 1)d$

so

$> \frac{n}{2} (2a + 14d)$

no we see that this equation has 2a +14d which is 2 times the first eq. so we can write

$= \frac{15}{2} (2)(40)$

$= 600$

Answer 2

Step-by-step explanation:

an=a+(n-1)d

a8=a+(8-1)d

40=a+7d

sum of 15 terms=n/2(2a+(n-1)d)

sum of 15 terms=15/2(2a+(15-1)d)

sum of 15 terms=15/2×2{a+7d}

sum of 15 terms=15/2×2×40

sum of 15 terms=15×40

sum of 15 terms=600