How to calculate sum of this series to the nth term???1+(2+3)+(4+5+6)+(7+8+9+...

1+(2+3)+(4+5+6)+(7+8+9+10)+(11+12+13+14+...
in class i saw a method to first just find the sum without the brackets,to the rth term,then find a connection between r and n with brackets.i dont fully understand so if some can help!!!!!
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