1）21/15×4/3+21/10×4/3-4/3
=（21/15+21/10-1）×4/3
=（7/5+21/10-1）×4/3
=（14/10+21/10-10/10）×4/3
=25/10×4/3
=5/2×4/3
=10/3
2）1*2/1+2*3/1+3*4/1+.+48*49/1+49*50/1
1*2=1/3*1*2*3
1*2+2*3=1/3*2*3*4
1*2+2*3+3*4=1/3*3*4*5
…………
1*2/1+2*3/1+3*4/1+.+48*49/1+49/*50/1
=1*2+2*3+3*4+.+48*49+49*50
=1/3*49*50*51
=41650