Question

the nth term of an ap is given by (-4+15).find the sum of first 20 term of this ap

Answer 1

hi friend...

this is what is given :

an = -4n + 15
put n = 1, we get
a1 = -4(1) + 15 = -4 + 15 = 11
put n=2, we get,
a2 = -4(2) + 15 = -8 + 15 = 7
now, d = a2 - a1 = 7 - 11 = -4
now, we have n = 20, so
S20 = 202[2(11) + (20 − 1)(−4)]    [as, Sn = n2[2a+(n−1)d]]      = 10[22 + 19 (−4)]      =10[22 − 76]      = 10 [−54] = −540....

hope this helps u...