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# ACT Math Concepts

### From Kaplan's 100 Key Math Concepts for the ACT - the ones that I didn't know :D

Question | Answer |
---|---|

Undefined Numbers | on the ACT, almost always means division by zero |

Imaginary Numbers | on the ACT, almost always means numbers that involve the square root of a negative number |

How do you solve a rates problem? | By using a proportion. |

What is the trick to finding the average of evenly spaced numbers? | Average the smallest and the largest numbers. |

How do you find possibilities? | Multiply the different elements factoring into the possibility by each other. |

How do you solve a system of equations | Multiply one of the equations so that when added to the other equation, either x or y cancels out. |

What is slope intercept form? | y=mx+b |

How do you find the slope-intercept of y or x? | Plug x or y (respectively) into the equation and solve. |

What is the equation for a circle? | (x-h)^2 + (y-k)^2 = r^2 |

Name the two special right triangles that correspond with the length of the legs. | 3-4-5 and 5-12-13 |

The sides of a 30°-60°-90° triangle are in a ratio of.... | 1:√3:2 |

The sides of a 45°-45°-90° triangle are in a ratio of.... | 1:1:√2 |

Formula for the area of a trapezoid | (b1+b2/2)xh |

The fundamental trigonometric identity: | sin^2x + cos^2x=1 |