an=6+3+5+9...+(n²-3n+5 )=6+【1²+2²...+(n-1)
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an=6+3+5+9...+(n²-3n+5 )=6+【1²+2²...+(n-1)²】-3【1+2+...(n-1)】+5(n-1)?
![an=6+3+5+9...+(n²-3n+5 )=6+【1²+2²...+(n-1)](/uploads/image/z/19765592-8-2.jpg?t=an%3D6%2B3%2B5%2B9...%2B%28n%26%23178%3B-3n%2B5+%29%3D6%2B%E3%80%901%26%23178%3B%2B2%26%23178%3B...%2B%EF%BC%88n-1%EF%BC%89%26%231)
an=6+3+5+9+...+(n²-3n+5 )=6+(2²-3*2+5)+(3²-3*3+5)+...+(n²-3n+5 )=6+(2²+3²...+n²)-3(2+3+...+n)+5(n-1)
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