Question

78,76,74,----------------------- 50 find the sum of terms in that AP

Answer 1

Answer:

a=78

d=76-78=-2

sn=?

sn=n/2(a+l)

firstly,

an=a+(n-1)d

50=78+(n-1)-2

50-78=(n-1)-2

-28=(n-1)-2

-28/-2=(n-1)

14=n-1

n=14+1

n=15

sn=n/2(a+l)

sn=15/2(78+50)

sn=15/2×128

sn=15×64

sn=960

Answer 2

Answer:

Hello Dear User__________

Here is Your Answer...!!

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Step by step solution:

$Given \ a=67 \ c.d. = 76-78=-2 \ and \ l=50\\\\we \ know \ formula\\\\l=a+(n-1)d\\50=78+(n-1)-2\\\\n-1=\frac{50-78}{-2}=\frac{-28}{-2}=14\\\\n=14+1=15\\\\For \ sum \ formula \ we \ have \ \\\\S_{n}=\frac{n}{2}[a+l]\\\\putting \ value \ here\\\\S_{15}=\frac{15}{2}[78+50]=\frac{15 \times128}{2}=15 \times64=960\\\\S_{15}=960$

Hope it is clear to you.