a-(a+b)+(a+2b)-(a+3b)+……-(a+101b)
=a+(a+2b-a-b)+(a+4b-a-3b)+...+(a+100b-a-99b)-a-101b
=a+b+b+...+b-a-101b
=50*b-101b
=-51b
=-51*-1/3
=17
a-(a+b)+(a+2b)-(a+3b)+……-(a+101b)
=a+(a+2b-a-b)+(a+4b-a-3b)+...+(a+100b-a-99b)-a-101b
=a+b+b+...+b-a-101b
=50*b-101b
=-51b
=-51*-1/3
=17