Question

Please solve it......

Answer 1

Sn = n/2{ 2a +(n-1)d} =72

n= number of terms
a= 1st term =17
d= common difference = -2

Let number of terms must be added to get sum 72 is V.

V/2×{ 2×17 + (V-1)× -2 } =72

V/2 × (34 -2V +2 )= 72
V× (36-2V) =144

36V - 2V^2 =144

2V^2 -36V+144 =0

V^2 -18V + 72=0

V^2 -12V -6V +72=0

V(V-12) -6( V-12)=0

(V-6)(V-12) =0

V=6,12

Either 6 Or 12 terms must be added to get sum of 72.


17,15,13,11,9,7 =72

17,15,13,11,9,7,5,3,1,-1,-3,-5 =72