Question

how many terms of the AP 1,4,7,........are needed to make the sum 51​

Answer 1

first term (a)=1

common difference (d)=3

we know

Sn=n/2 {2a+(n-1) d}

=>51=n/2 {2.1+(n-1) 3}=n/2 {3n-1}

=>3n^2-n=102

=> 3n^2-n-102=0

use quadratic formula

n=6,-17/3

but n =-17/3 is not possible

so, n=6

Answer 2

Answer:

Step-by-step explanation:

first term (a)=1

common difference (d)=3

we know

Sn=n/2 {2a+(n-1) d}

=>51=n/2 {2.1+(n-1) 3}=n/2 {3n-1}

=>3n^2-n=102

=> 3n^2-n-102=0

use quadratic formula

n=6,-17/3

but n =-17/3 is not possible

so, n=6