Question

tìm n thuộc N*

a)2+4+6+...+2n=210

b)1+3+5+...+(2n-1)=225

Answer 1

a) 2 + 4 + 6 + ... + 2n = 210

1.2 + 2.2 + 2.3 + ... + 2n = 210

2.(1+2+3+...+n) = 210

1 + 2 + 3 + ... + n = 105

\(\frac{n\left(n+1\right)}{2}=105\)

n(n+1) = 210

n(n+1) = 14.15

=> n = 14

Answer 2

câu b quắp

Answer 3

b)1+3+5+…+(2n-1)=225

(2n-1+1)n/2=225

2n^2/2=225

=>n^2=225

225=15^2

=>n=15