Question

The nth term of an A.P is given by (-4n+15). Find the sum of first 20 terms of this A.P.

Answer 1

• n =1
• T1 = a = -4×1 + 15
= 11

• n=2
• T2 = a+d = -4×2+15
= 7

• a = 11
• d = T2-T1 = -4

• Sn = n/2 (2a+(n-1)d
• S20 = 20/2 (2×11 + (20-1)×-4)
= 10 (22+(-76)
= 10 × -54
= -540


hope this helped you ...........!

Answer 2

put n=1,2,3....
by simple method
 (-4×1 + 15) + (-4×2 + 15) + ... + (-4×20 + 15) 
= -4 (1+2+...+20) + (15×20) 
= -4×20×(20+1)/2 + 300 
= -840 + 300 
= -540