1)7 1/2*6 1/2=(7+1/2)(7-1/2)=7²-(1/2)²=
49-1/4=48+4/4-1/4= 48 3/4
2)11 3/4*12 1/4=(12-1/4)*(12+1/4)=
12²-(1/4)²=144-1/16=143+16/16-1/16=
143 15/16
3)100²-99²+98²-97²+...+4²-3²+2²-1=
(99+1)²=99²+2*99+1-99²=199
(97+1)²-97²=97²+2*97+1-97²=195
(95+1)-95²=2*95+1=191
(5+1)²-5²=2*5+1=11
(3+1)²-3²=2*3+1=7
2²-1²=3
(199+3)+(195+7)+(191+11)+ ....=
202+202+202+.....+..=
202*50=10100 ;(200:4=50)