The 5th & 15th terms of ap are 13 & -17 respectively find the sum of the first 21 terms of the ap

The 5th & 15th terms of ap are 13 & -17 respectively find the sum of t

1.	The 5th & 15th terms of ap are 13 & -17 respectively find the sum of the first 21 terms of the ap
Answer» Given,A+4d=13 ___(1) and a+14d=-17___(2)Subtracting (1) from (2):-A+14d=-17A+4d=13- - -__________10d=-30__________D=-3Putting this value in eq.(1):-A+4d=13A+4(-3)=13A-12=13A=25Sn=n/2 [2a+(n-1)d]S²¹=21/2 [225 +(21-1)-3]S²¹=21/2 [50 - 60]S²¹=21/2 *(-10)S²¹=-105