50*50-49*49+48*48.+2*2-1*1

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50*50-49*49+48*48.+2*2-1*150*50-49*49+48*48.+2*2-1*150*50-49*49+48*48.+2*2-1*150*50-49*49+48*48.+2*2

50*50-49*49+48*48.+2*2-1*1
50*50-49*49+48*48.+2*2-1*1

50*50-49*49+48*48.+2*2-1*1
50*50-49*49+48*48.+2*2-1*1
=(50+49)(50-49)+(48+47)(48-47)+-----+(2+1)(2-1)
=50+49+48+47+-------+2+1
=(50+1)*50/2
=1275

50^2-49^2+48^2-47^2……+4^2-3^2+2^2-1^2
=(50+49)(50-49)+(48+47)(48-47)+(46+45)(46-45)+……+(2+1)(2-1)
=99+95+91+……+3
=(99+3)×25/2
=51×25
=1275