Respuesta :
Answer:
n=10
Step-by-step explanation:
The given equation is
[tex]10+12+14+...+2n=90[/tex] ...(1)
Let nth term is 2n.
We need to find the values of n.
It is clear that [tex]S_n=10+12+14+...+2n[/tex] is sum of A.P., whose first term is 10 and common difference is 2.
Sum of A.P. is
[tex]S_n=\dfrac{n}{2}[2a+(n-1)d][/tex]
where, a is first term and d is common difference.
Substitute a=10 and d=2 in the above formula.
[tex]S_n=\dfrac{n}{2}[2(10)+(n-1)2][/tex]
[tex]10+12+14+...+2n=n[10+n-1][/tex]
[tex]10+12+14+...+2n=n[9+n][/tex] ...(2)
From (1) and (2), we get
[tex]n(9+n)=90[/tex]
[tex]n^2+9n-90=0[/tex]
Splitting middle term, we get
[tex]n^2+15n-6n-90=0[/tex]
[tex]n(n+15)-6(n+15)=0[/tex]
[tex](n+15)(n-6)=0[/tex]
[tex]n=-15,6[/tex]
Since, n represents the number of terms so n cannot be a negative number, therefore number of term is 6.
Note: nth term and variable n both are different.
[tex]a_n=a+(n-1)d[/tex]
[tex]a_6=10+(6-1)2=10+10=20[/tex]
Sixth term is 20. So,
[tex]2n=20[/tex]
[tex]n=10[/tex]
Therefore, the value of n is 10.
