buktikan bahwa 2+4+6+... +2n=n(n+1)

2+4+6+...+2n=n(n+1)

misal n=1
2n=n(n+1)
2(1)=1(1+1)
2=1(2)
2=2 (benar)

misal benar n=k
2+4+6+...+2n=n(n+1)
2+4+6+...+2k=k(k+1)

misal n=k+1
2+4+6+...+2k+2(k+1)=k(k+1) + 2(k+1)
=k^2+k+2k+2
=k^2+3k+2
=(k+1) (k+3)
=(k+1) ((k+1)+1)