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Write the expanded form of (5a + 6b + 8c) 2 .
25a┬▓ + 36b┬▓ + 64c┬▓ + 64ab + 96bc + 80ac
25a┬▓ + 36b┬▓ + 64c┬▓ + 60ab + 91bc + 80ac
25a┬▓ + 36b┬▓ + 64c┬▓ + 60ab + 96bc + 80ac
25a┬▓ + 36b┬▓ + 64c┬▓ + 60ab + 96bc + 90ac
25a┬▓ + 36b┬▓ + 64c┬▓ + 60ab + 96bc + 80ac
Use the symmetry of the identity: The first three terms are squares of the components (25a2,36b2,64c2). The remaining terms are twice the products of the pairs (2├Ч5a├Ч6b=60ab,2├Ч6b├Ч8c=96bc,2├Ч5a├Ч8c=80ac).
Expression to expand: (5a + 6b + 8c)2
(x+y+z)2=x2+y2+z2+2xy+2yz+2zx
Use the symmetry of the identity: The first three terms are squares of the components (25a2,36b2,64c2). The remaining terms are twice the products of the pairs (2├Ч5a├Ч6b=60ab,2├Ч6b├Ч8c=96bc,2├Ч5a├Ч8c=80ac).
Students often forget to multiply the mixed terms by 2 (e.g., writing 30ab instead of 60ab) or miss the final term (2zx).
Identify terms in the formula
Map the components of the expression (5a+6b+8c)2 to the identity (x+y+z)2=x2+y2+z2+2xy+2yz+2zx, where x=5a, y=6b, and z=8c.
x=5a,y=6b,z=8c
Calculate square terms
Compute the squares of individual terms: (5a)2=25a2, (6b)2=36b2, and (8c)2=64c2.
25a2+36b2+64c2
Calculate product terms
Compute twice the cross-products: 2(5a)(6b)=60ab, 2(6b)(8c)=96bc, and 2(5a)(8c)=80ac.
60ab+96bc+80ac
Combine result
Summing all the parts gives the final expanded form 25a2+36b2+64c2+60ab+96bc+80ac.
25a2+36b2+64c2+60ab+96bc+80ac
C is correct because the expansion of (5a+6b+8c)2 according to the identity results in 25a2+36b2+64c2+60ab+96bc+80ac.
This identity is a foundational block for completing the square in quadratic equations and understanding distance formulas in 3D coordinate geometry.