Compound Functions

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Priyaranjan: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Wed Mar 04, 2015 5:16 pm; Location: India; Thanked: 1 times

by Priyaranjan » Thu Nov 26, 2015 9:39 pm

Q. If g(x)= 3x+âˆšx, what is the value of g(d^2+6d+9)

Ans. Can I solve this way?

d^2+6d+9= (d+3)(d+3)= d= -3

g(-3)= 3(-3)+âˆš-3= -9-âˆš3.

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Source: Beat The GMAT — Problem Solving | Thu Nov 26, 2015 9:39 pm

Matt@VeritasPrep: GMAT Instructor; Posts: 2630; Joined: Wed Sep 12, 2012 3:32 pmFollowed by: 119 members; Location: East Bay all the way; Thanked: 625 times; GMAT Score:780

by Matt@VeritasPrep » Thu Nov 26, 2015 11:15 pm

You can't, because you don't know that dÂ² + 6d + 9 = 0. You could, however, say that

g(dÂ² + 6d + 9)

= 3(dÂ² + 6d + 9) + âˆš(dÂ² + 6d + 9)

= 3dÂ² + 18d + 27 + âˆš(dÂ² + 6d + 9)

But we'd need more info from here.

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