用归纳法证明0.2+1.3+2.4+3.5+.+(n-1).(n+1) = n(n-1)(2n+5)/6
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用归纳法证明0.2+1.3+2.4+3.5+.+(n-1).(n+1) = n(n-1)(2n+5)/6
用归纳法证明0.2+1.3+2.4+3.5+.+(n-1).(n+1) = n(n-1)(2n+5)/6
用归纳法证明0.2+1.3+2.4+3.5+.+(n-1).(n+1) = n(n-1)(2n+5)/6
n=1,显然成立,略
假设n=k成立,k≥1
即0*2+1*3+……+(k-1)(k+1)=k(k-1)(2k+5)/6
则n=k+1时
0*2+1*3+……+(k-1)(k+1)+k(k+2)=k(k-1)(2k+5)/6+k(k+2)
=k(2k²+3k-5+6k+12)/6
=k(2k²+9k+7)/6
=k(k+1)(2k+7)/6
=[(k+1)-1](k+1)[2(k+1)+5]/6
所以n属于正整数时,0*2+1*3+……+(n-1)(n+1)=n(n-1)(2n+5)/6
very good. standard answer.
当n=1时，(n-1)(n+1)=n(n-1)(2n+5)/6=0
当n=2时，0.2+1.3=3 n(n-1)(2n+5)/6=3
0.2+1.3 =n(n-1)(2n+5)/6
当n>=2时，假设0.2+1.3+2.4+3.5+......+(n-1).(n+1) = n(n-1)(2...
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当n=1时，(n-1)(n+1)=n(n-1)(2n+5)/6=0
当n=2时，0.2+1.3=3 n(n-1)(2n+5)/6=3
0.2+1.3 =n(n-1)(2n+5)/6
当n>=2时，假设0.2+1.3+2.4+3.5+......+(n-1).(n+1) = n(n-1)(2n+5)/6成立
则，0.2+1.3+2.4+3.5+......+(n-1).(n+1)+n（n+2）
=n(n-1)(2n+5)/6+n（n+2）
=n(n+1)[2(n+1)+5]/6
故证
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