b) VT = (7a-3b)2 - 4c2 = 49a2 - 42ab + 9b2 - 4c2
mà 10a2 = 10b2 + c2 nên c2 = 10a2 - 10b2
nên VT = 49a2 - 42ab + 9b2 - 4 (10a2 - 10b2)
=49a2 - 42ab + 9b2 - 40a2 + 40b2
=9d2 - 42ab + 49b2 = (3a - 7b)2 = VT
Cho 10a^2 = 10b^2 + c^2
CMR: ( 7a - 3b + 2c )( 7a - 3b - 2c ) - (3a - 7b )^2
cho: 10a^2=10b^2+c
tinh (7a-3b+2c).(7a-3b-2c)=(3a-7b)^2
Cho 10a^2=10b^2+c^2. Chứng minh rằng (7a-3b+2c)(7a-3b-2c)=(3a-7b)^2
Cho 10a^2= 10b^2-c^2
CMR ( 7a-3b-2c)(7a+3b+2c) = (3a-7b)^2
Cho 10a^2=10b^2+c^2. Chứng minh rằng (7a-3b+2c)(7a-3b-2c)=(3a+b)^2
cho \(10a^2\)=\(10b^2\)+ \(c^2\)
chứng minh rằng : (7a - 3ab + 2c ) (7a - 3b - 2c ) = (3a - 7b )\(^2\)
cmr: (a+2b-3c)^3+(b+2c-3a)^3+(c+2a-3b)^3=3.(a+2b-3c).(b+2c-3a).(c+2a-3b)
1) Rút gọn :
\(B=\frac{\left(a+2b\right)^3-\left(a-2b\right)^3}{\left(2a+b\right)^3-\left(2a-b\right)^3}:\frac{3a^4+7a^2b^2+3b^4}{4a^4+7a^2b^2+3b^4}\)
Cho a+b+c = 1 và 3a+2b>c, 3b+2c>a, 3c+2a>b. Chứng minh: 1/(3a+2b-c) + 1/(3b+2c-a) + 1/(3c+2a-b) >hoặc = 9/4
