10*11+11*12+.+14*15
=10*(10+1)+(10+1)(10+2)……+(10+4)(10+5)
=(14-10+1)10*10+(1+1+2+2+3+3+4+4+5)*10+1*2+2*3+3*4+4*5
=500+250+40
=790
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