Tính nhanh các phép tính : ( Ghi rõ cách giải )
a) 1/10 + 1/40 + 1/88 + 1/154 + 1/138 + 1/340
b) 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + … + 1/110
Tính nhanh các phép tính : ( Ghi rõ cách giải )
a) 1/10 + 1/40 + 1/88 + 1/154 + 1/138 + 1/340
b) 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + … + 1/110
a,
1/10+1/40+1/88+1/154+1/138+1/340
=1/2×5+1/5×8+1/8X11+1/11×14+1/14×17+1/17×20
=1/3x(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20)
=1/3x(1/2-1/20)
=1/3×9/20=3/20
b,
b = 1/2 + 1/6 +1/12 + 1/20 + 1/30 + 1/42 + … + 1/110
b = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/ 6.7 +…+ 1/10/110
b = 1 – 1/2 +1/2 – 1/3 +1/3 -1/4 +1/4 – 1/5 + 1/5 -1/6 + 1/6 -1/7 +…+ 1/10 -1/11\
b = 1 – 1/11
b = 10 /11