(tga+ctga) : (tga - ctga) = -1 / 2cos²a - 1 = -sec(2a)
1)
7 3/20-5 1/12=7 9/60-5 5/60=
=2 4/60=2 1/15
2 1/15×1.5=2 1/15×1 1/2=
=31/15×3/2=31/10
6 3/7×1.4=6 3/7×1 2/5=
=45/7×7/5=9
9÷1.8=5
31/10÷5=31/10×1/5=31/50
2)
5.7÷0.3=19
19-8 1/3=18 3/3-8 1/3=10 2/3
10 2/3×0.75=10 2/3×3/4=
=32/3×3/4=8
2 1/8+1.75=2 1/8+1 3/4=
=2 1/8+1 6/8=3 7/8
3 7/8÷15.5=3 7/8÷15 1/2=
=31/8÷31/2=31/8×2/31=1/4
8÷1/4=8×4/1=32
4*17=68 .......................
4т5ц6кг+18ц34кг=4т23ц40кг