Cho \(a,b>0\). Chứng minh rằng:
\(\dfrac{1}{ab}\ge\dfrac{4}{\left(a+b\right)^2}\)
giúp mik zs mik ngu toán lém

1. At last you're here! I(wait)................... here for more than half an hour!
2. My father(work)................ in Canada for last year, so by the time he(return)...................... the month after next I(not see)................ him for fourteen months.
3. When you(see)............. him again you(be)..........surprised to see that his health(improve)............ since he(go) to Switzerland.

Một hình chữ nhật có chu vi 320m. Nếu tăng chiều rộng 20m, chiều dài 10m thì diện tích tăng 2700m². Tính diện tích ban đầu.