Question

51+50+49+ ..... + 21= .....

• A. 1116
• B. 1122
• C. 1128
• D. 1124

Answer 1

Answer:

A. 1116

$a_1=51,d=-1$

$\text{അവസാന പദം}, a_n=21$

$n = [(a_n - a_1)\div{d}] + 1$

$n=((21-51)/-1)+1 =31$

$S_n=n/2\times[2a+(n-1)]\times{d}$

$=31/2\times(2a+30d)$

$=31/2(102-30) =1116$