Question

2+4+6+8+.....+n=240​

Answer 1

Step-by-step explanation:

2+4+6+....+ n = 240

By AP,

Sn = n/2[2a+ (n-1)d]

= 240 ×2 = n[4+(n-1)2]

= 480 = n( 4+ 2n -2)

= 480 = 2n² +2n

= n² + n - 240 = 0

= n² + 16n -15n -240 =0

= n(n+16) -15(n+16) = 0

= (n+16)(n-15) = 0

hence, n= -16 and 15