Sai thì thôi nha :>

Theo đề ra: ABCD là hình bình hành

\(vectoAD+vectoAB=vectoAC\) và \(vectoAB+vectoAC=vectoBD\)

\(\Leftrightarrow vectoAD=-vectoAB+vectoAC\)

\(\Leftrightarrow vectoAD=vectoAB+vectoAC-2vectoAB=vectoBD-2vectoAB=b-2a\)

\(\overrightarrow{BD}=\overrightarrow{BA}+\overrightarrow{BC}=-\overrightarrow{AB}+\overrightarrow{AD}\)

\(\overrightarrow{AC}=\overrightarrow{AB}+\overrightarrow{AD}\)

suy ra \(2\overrightarrow{AD}=\overrightarrow{BD}+\overrightarrow{AC}\Leftrightarrow\overrightarrow{AD}=\frac{\overrightarrow{BD}+\overrightarrow{AC}}{2}=\frac{a+b}{2}\).

Ta có \(D=sin^2a-cosa-1=-cos^2a-cosa=-\left(cos^2a+cosa+\frac{1}{4}\right)+\frac{1}{4}\le\frac{1}{4}\)

mình đang học onl nên là rep muộn chút

Đặt \(sina=x;cosa=y\)ta có : \(x^2+y^2=1\)

Khi đó : \(-E=x^2+y^2-x-y-1=\left(x-\frac{1}{2}\right)^2+\left(y-\frac{1}{2}\right)^2-\frac{3}{2}\ge-\frac{3}{2}\)

\(< =>E\le\frac{3}{2}\)

sai thì thôi nhé 

. Daisy, Anna, ………………tall for her age

A.am          B.IS           C.ARE

Hok tốt

b nha bn

1.“Who……………………….that horrible noise?” “ It’s Tom pracing the violin” 

A. makes        B. made        c.is making

11………………………the moon last night?

A.have you been               B.DId you see              C.Do you see

2. My uncle had the telephone…………………..out of his office.

A.take        B.taken       c.taking

1.A

2.B

3.C

4.A

Học tốt

1.That .............. in 1945.

A.was happened          B.happened    

2. We visited a college……………….in 1890.

A.find                    B. found

3.The boy suffered …………………. a very bad headache. 

A.of            B. From              C.by

4. Thunder often………………small girls.

A.fears                    B.frightens                              C.Frightenings