Question

Solve the equation: 1+4+7+...+x=715.

Answer 1

Step-by-step explanation:

It is looking like an AP so i will aply AP formula

1/2(ft+Lt)=715

x= 1530-1=1429

Answer 2

Answer:

the value of x is 64

hope it helps you thanks

Step-by-step explanation:

This is an AP where a=1 and d=3

a n = a +(n-1)d=x

a n=1+(n-1)3

a n=1+3n-3

a n=3n-2

3n-2=x .......(1)

Sn=n/2(a+l) where a=first term's number and l=last term's number ,here x ie. 3n-2 is the last term's number

715×2=n(1+3n-2)

715×2=3n^2-n

3n^2-n-1430

n={1+ root [1-4×3×(-1430)]}/2×3 and

{1-root[1-4×3×(-1430)]}

n =(1+131)/6 and (1-131)/6

n=132/6 ,-130/6

so n=22 ( as we can't take n as negative number)

so last term of the given AP is 22 and number of the last term ie. x is

x= 3n-2 ........taking (1)

x=3×22-2=66-2=64